Eli5 Layer 2 and how to use

Alright so I've been in ETH for a while so I understand it for the most part. Just haven't committed any time to learning about polygon and other layer 2 solutions.

What is necessary to interact with them and are they currently resulting in savings in eth txns?

For example if I wanted to swap ETH for another ERC-20 token on MATIC what would be the process and how much would I save?

Thanks.