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

Question

Earlier today after one of my games, the game awarded me a bonus 3000 experience with the description "freshness bonus". Under what circumstances does the game award this bonus?

Answer 1

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.

Answer 2

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)}