Think of an email address. Accessing a community is like accessing an email address (that’s why communities have handles @ a server.
So unlike Reddit where you could have r/memes in the Fediverse you can have [email protected] or [email protected] each host can host a bunch of communities and a bunch of users and we all connect to one another in a similar way that email works.
The plus side of this is that no single company has all the power like a walled garden, rather the whole system functions in a decentralised manner. It is also run open source by a community of developers. So while the whole system works in a very similar way to Reddit, you cant just search for the official memes community, rather you can subscribe to many, some may be large and some may be small and niche. Hopefully the downsides of this will get ironed out, organising communities into super communities or sorting by tag or something, but on the bright side, being open source and decentralised, development of Lemmy will likely proceed at a rapid pace and soon catch up and overtake corporate sites in useability, as they increasingly look to stifle useability and freedom for profit.