The letter grade at end of game.

Technically, it gives you more mastery points.

In reality, it's only to let you know how you did generally because it's a consequence of the points and not a cause.

it just tells you how many mastery points you got that round.

S+ is the general most you can get. F- is the general least you can get.

I want to use these before they go away so, [slayer-pantheon-rainbows]

I've heard it's to compare your gameplay of that champion to the general gameplay of others to that champion. I don't know if that's true.

It's not quite accurate because it's based off of things like creep score and whether you upgraded your trinket or not (and of course other things) so you see inaccuracy patterns such as supports who get an A- when in reality, they played better than their S adc.

With the upcoming chest and keys feature, chest quality will be determined by mastery grade. Not sure how though.