Sure, it's when you see the feed of people you follow. Right now, most clients will show the latest notes made by those people, and as you scroll, you see older and older notes.

That's one kind of algorithm called "reverse chronological". Primal and some other clients have a sidebar of "most zapped" and such notes that provides an alternative algorithm to see what's popular or trending.

Centralized algorithms like Twitter or Facebook will lead you to believe you're viewing reverse chronological, but they have an interest in picking and choosing what you see to keep you engaged on their platform. This can't happen on Nostr unless you follow a relay that does that. But if you don't like that relay, you have the option to drop it and use a more neutral one. I think that's the biggest way Nostr, in it's decentralization, gives power back to you and me, the users.