[adeptus]

https://en.wikipedia.org/wiki/Mudra

It's a fairly comprehensive write-up. I'm mostly familiar with the Hatha Yoga context of mudras.

There's also a few dozen books on the topic. I'd stick to the older more original texts if you want a real understanding, and stay away from the more modern new age fluffy bunny "crystal mudras" nonsense.

[adeptus]

Forgot to provide the link to the books: https://www.amazon.com/s?k=Mudras&ref=nb_sb_noss