Q4All: What does settling even mean?

There's a ton of stuff on here discussing whether people "settle" and why or why not it's bad to.

So, it's supposed to mean someone chooses to be in a relationship with someone who isn't their ideal partner? But, who does this actually happen to?

As a man, would I be settling with any girl who wasn't a rich model who's completely devoted to me even if I treat her selfishly? Everyone has to "settle" if they want to realistically have a relationship. I doubt there's many people who actually think their partner couldn't improve in any way.