I only heard about them recently too so I might give an incomplete answer but the general gist is that mutual aid is when a group of people band together and share whatever resources and services that they have to offer to other people in that group.

So if someone made an excess of vegetables in their garden they would give that to others in the group with expecting anything immediately in return in the hopes that when their fence breaks down and they request help someone with knowledge on how to fix it would be willing to come help.

As for finding mutual aid groups I’ve seen mutualaid.wiki and mutual aid hub but I’m not sure of what else there is.