What does "freshness bonus" mean and how do I get it?

In junction to @Vemonus's answer, the wiki adds a bit different wording plus a nice graph.

A Freshness Bonus in Splatoon 2 refers to the proficiency you've earned for each individual weapon. This is different from Judd's coin bonus from the original game.

Each weapon has a freshness rating that will improve every time you win a match. It starts at 0 and goes up one point each time you win a match. Losing a match will subtract from your score, but how much it subtracts depends on your current rating. Once you reach a certain amount, a point bonus is applied.

Take note that while your ranking will continue to go up, the highest you can go is SUPERFRESH! along with all three one time bonuses.

This Twinfinite article explains the Freshness bonus pretty well:

Your Freshness Rating is all about bonuses and is tied to two things: your weapon and your ability to win matches. If you can excel at winning and using a specific weapon, you can earn a Freshness Bonus which is a large amount of experience points to help you level up faster.

Each weapon has its own rating, so if you want to build the rating you need to stick to a gun, preferably one you’re really good with. While using that gun, its rating will go up every time you win a match by .5 points, but if you lose it can go down a full point. So you need a positive win/loss ration to rank up. If you happen to rank up a weapon during a match, when it’s time to get your rewards you’ll see your Freshness Bonus as a huge splat next to your normal points and it’s always a few thousand.

_{(emphasis mine)}