For general understanding maybe Wikipedia?
For Bitcoin specific applications
https://medium.com/tx-fusion/zero-knowledge-proofs-bitcoin-l2-solutions-a8d913ceac4e
Zero-knowledge proof - Wikipedia

The Bitcoin Manual
What Are Zero-Knowledge Proofs? - The Bitcoin Manual
An introductory look at a proposed scaling solution for bitcoin known as ZK rollups, what it does, how it works and where it would fit into the net...
validity rollups on bitcoin