א שיינעם דאנק פור די שאלה!

Yiddish is, as the podcast and others have said, closely related to German. It's got loans from Hebrew, Aramaic, Slavic languages, and occasionally romance loans (beyond what exists in German dialects), and it's got pronunciation differences like a German dialect would. It formed by Jews in German-speaking areas migrating east but still speaking Yiddish, forming eastern Yiddish, while those that stayed had their language diverge from German and spoke western Yiddish, which is extinct.

Measuring mutual intelligibility is difficult. I've seen lots of German-speakers say they can figure out Yiddish, but usually that's standard Yiddish on YouTube or something. Colloquial Yiddish has lots more loans from other languages and can have funkier vowel shifts than standard Yiddish. Yiddish speakers can't always understand each other!

It's also easy to construct a Yiddish sentence unreadable to Germans because of vocabulary. Like "der rov leynt a seyfer" means "the rabbi reads a book", but rov, leynt, and seyfer are not German words. Or an actual example from /r/Yiddish, "kulem hobn nekudes toyves", meaning "everybody's got good points"--all words but "hobn" are loans

An episode of the AskHistorians podcast deals with this topic. I've pinged /u/gingerkid1234 in case he has more to add.

I have read that it is related to German, and i have also read (although i could certainly believe otherwise) that one who speaks german can easily understand Yiddish. Is there any truth to that as well?

This is just my personal experience ...

Spoken Yiddish indeed always seemed like a weird German dialect to my Austrian-German ears. I don't know a lot about it from either the historic or linguistic perspective, but Dutch as another example for a language related to German, is a lot harder to decipher for a German native speaker.

On the one hand, Yiddish seems to borrow a lot from German, but there are also words that clearly found their way into the German language, many only used in slang, via Yiddish: Maloche, Schlamassel, meschugge, Chuzpe, koscher, ...