What does gc-interval in named sets actually do?

Question

I'm testing out named sets in my nftables ruleset but I can't find any explanation of what the gc-interval parameter actually does.

The documentation simply states:

Garbage collection interval, only available when timeout or flag timeout are active.

But what exactly does garbage collection mean in this context?

A wiki example uses it like so:

set example {
  type ipv4_addr
  flags timeout
  timeout 10s
  gc-interval 4s
}

So what does the gc-interval 4s do in relation to the timeout 10s?