It's fairly different, but recognizably related to it. About 70% of the words in Yiddish are of Germanic origin, the rest being mostly from Hebrew & Aramaic and from Slavic languages. This immediately sets it apart and can inhibit mutual comprehension.
Some of the Germanic words are also used differently or have grammatical differences in comparison with Modern German (e.g. Nouns may have different genders, verbs may be conjugated differently etc). German form plurals mostly with -n or -er (besides vowel changes). Yiddish has a plethora of possible endings besides, such as -s, -im, -es, amongst others.
Both German and Yiddish went through a variety of phonetic changes since they parted ways. Yiddish doesn't have umlauts, it's /kh/ sound is always as in German /ach/, never as in /ich/. Old High German /p/ changed to /pf/ in German, but not in Yiddish and voiced consonants at the ends of words in Yiddish are not devices as in German. For example, German Apfel is epel in Yiddish.
German word order changed in the Middle Ages so that the verb is sometimes at the end of the sentenced. In Yiddish, the verb is always in the second position in the sentence.
From what I've observed, Germans can understand some Yiddish if both sides try to accommodate each other. Yiddish speakers have much more trouble understanding German. It's certainly possible to come up with sentences that would be understood by both groups, but you'd need to know both languages quite well to be able to construct such sentences.https://www.quora.com/How-much-is-Yiddish-understood-by-standard-German-speakers
That depends rather on the variety of Yiddish. Some dialects, such as the Lithuanian or Eastern Russian varieties, are not that different from Hochdeutsch and can be made more “Germanlike” (there is an older, journalistic style of Yiddish that deliberately tries to be more “German” and uses imitation German expressions that are less common in the spoken language). Germans can often follow this variety of Yiddish to some extent despite the odd accent and strange grammar (as well as Hebrew vocabulary and idioms, as well as Slavic borrowings).
Polish and Hungarian Yiddish, however, will be much more challenging. The pronunciation is much stranger and the grammar has evolved quite a bit more away from Hochdeutsch. Speakers of different Yiddish dialects can usually understand each other fine but a German might be jyst a bit too removed to keep up. Idioms and the way the language is used also cause problems.
Yiddish proverbs and idioms are a big feature and they are not going to mean a thing to a gentile German.
For example, whether one says it “chop nit di lokshn forn fish” (Lithuanian/Russian Yiddish) or “chup zhe nisht di lukshn fur di fish” (Polish) a German is not going to immediately know what “don’t grab the noodles before the fish” means.
The meaning is “don’t get ahead of yourself” or “don’t try to walk before you can crawl”. The noodles and fish reference comes from the classic Ashkenazi shabbes dinner on Friday night. We begin the meal, traditionally, with a fish appetiser and then move on to a yoich (soup) typically chicken noodles (thus the noodle reference) but, if you are lucky, with kreplach or matzo balls (I love kreplach the most and would rather have them, or matzo balls, than lokshn any day!) . Then there is usually a meat dish as the main course but why mention that when you haven’t even had your fish, nu?
İdioms pepper Yiddish speech, including full Torah quotations in Hebrew (with our mangled Ashkenazi pronunciation so that even Israelis have no idea what we are on about!).
For example, here is a short dialogue in Yiddish (Litvak/Russian) for any German speaker to try their hand at.
“Nu, Reb Moyshe! Vos machstu do? “
“Reb Mendel! Ich leb azoy, borech haShem! halevai veiter! Un du, vos machstu? “
“Ach! Ich bin an alter kaker! Ich dertzeyl dir nit meine tzores! Di hert nor der eybishter, HaShem borech hu! “
“Tzores? Voser tzores? Tzi hostu nit keyne simches? Mein veibl hot gezogt es is geven a chassene bei eich! ”
“Yo, mamesh, Mein yinger zun Velvele hot zich gechasent”.
“Mazel Tov! A broche of dein Simche! Vos machn di naye Chosnkale? “
“Borech HaShem, mir hobn gemacht a gut shidech! Zey zenen zeyer freylech tzuzamen! Un bei dir reb Mendel? Vi bistu geven? Ich Ze dich nit mer inem koylel. Vu lernstu in di teg? “
“Ach, du host recht Reb Moyshe! Ich bin broygez ofn rovn! Ich kum nit mer tzu dem koylel! “
“Ober farvos? Vos hot der rov gemacht azoy? “
“Ich vil nit dertzeyln loshn hore! Chas vechalile! Ich zog nor mir trachtn nit mitn zelbikn kop. Ich lern nu bei di Breslevers”.
“Di Breslevers? Di ‘na-nach’ meshugene freaks? Di zenen ale shikere chassidim un nit keyne talmidim chachomim! “
“Ich hob dos shtibl lib! Zey zenen gute yidn un mir tantzn dortn a sach. Mir lernen dortn oych mit simche. “
“Ach di Breslevers? Az ich volt dos nit fun dein moyl gehert, volt ich nit keynmol gegloybt! “
“Yo, Ober dos is der emes! Zei nit broygez of mich mein alter chaver! S’is aza dos lebn! Ich bin dortn gantz freylech. “
“Nu. Azoy, Zei gezunt! “
“Zei gezunt! “
The setting is a typical religious one, because it is mainly among very devout, traditional Jews that the language lives these days. One thing I did not include in my little imaginary dialogue between Moyshe and Mendel was a bit of English--and that is actually a significant omission in modern Yiddish. The religious context also ups the Hebrew content of the discussion a bit, but there is no avoiding the fact that Yiddish is peppered with hebraicisms in every register.