Gonna give it a shot, so you know how there is reddit and it has communities and those communities have posts which have comments etc. Now imagine there are 2 reddits, Addit and Bddit both are exactly same in infrastructure/software stuff but obviously different users, communities and content but since the infrastructure is the same, both Addit and Bddit decided that it would be cool if user@addit could interact and do reddit stuff on Bddit and vice versa. The software that does this is lemmy, anyone can make a reddit like website with this and all users of that website will be able to interact with every other lemmy website. So lemmy is a group pf reddit. Now as you can imagine, website owner can decide which other website can interact with their website, this interaction is called "federating" so defederation means that blocking a certain website (also reffered as lemmy instance), as an example, addit could block bddit or vice versa so users of addit can not interact with bddit.

Now the concept of fediverse is also similar, in that case fediverse is a goup of fedi services, think a group of lemmy like services where each service has a group of websites/instances. This is possible because most of these services have similar things inside them, like each will have users, posts, comments etc so user@lemmy_instance@lemmy can interact with user@mastodon_instance@mastodon. All this is govern by ActivityProtocol. Honestly i don't know to what extent these interactions are possible.

Sooooo in short lemmy= network/group of reddit like websites Fediverse= network/group of lemmy like services