Computer nerd lingo for talking about complexity.

O(n) means "on the order of n operations" or "on the order of n amount of space" in the worst case.

O(n^2) mean "on the order of n^2 operations" etc. etc.

Where n is the number of input elements.

For instance, the "bubble sort" algorithm, when given n elements, takes "on the order of" n^2 operations to complete, or O(n^2).

In the case of Nostr, the number of connections in a social graph is "on the order of" n relays * m users. In reality it's not actually that bad, but that is an upper bound for how bad it "could" be.

It's known as "big-O" notation. There's also "little-o" notation. And omega, and others, for those inclined.