For psychology purposes, a topic's salience is how much of one's attention it gets.

You've surely met someone who can't seem to shut up about a certain topic. Maybe it's a ten-year-old who just LOVES Pokémon, maybe it's a grown man who's idolized some populist politician. Every conversation with this person somehow gets redirected to the thing they want to talk about, and it's always the same thing. That topic has a high salience for that person. Psychologists will be on the lookout for people who can't seem to give the proper amount of attention to other topics or activities, such as their family or personal hygiene, which is a bright line between "oh lawd, he on THAT again..." and "cray-cray harmful behavior."