From left to right: it allows you to toggle on/off relays for:

Home feed (posts from people you follow)

DMs

Channels/Groups (not sure about this one)

Global feed (all notes posted to relays you've enabled)

Down arrow: # of Notes read from this relay

Up arrow: # of Notes written to this relay(this is grey if it's a paid relay& you're not subscribed to it)

Error count

Spam count