It basically means how much an issue is on people’s minds and how much an issue has on public opinion. As an example, in the 2012 election, health care was a high salience issue since the candidates talked about it a lot. In 2016, immigration had increased salience- it wasn’t talked much about in 2012 but Trump’s nomination and “build the wall” increased the salience. In 2020, immigration was lower salience since Covid and Black Lives Matter took up most of the dialogue.

Edit: I’m not sure if this fits your context. I’ve only ever heard the word with respect to politics.