What is pip?

Pips are an indicator of how close you are to your next rank up.

Here you can see this endgame screen, which you've no doubt seen several times since it pops up every match.

This determines how much rank progress you'll get, as it says on the side. It fills this bar based on how good your Emblems are at the end of the match.

The black diamond about halfway through is a Safety Pip. It means you get no pips, but you don't lose any pips either - if you score below it, you lose a pip.

The white diamond is a regular pip. For reaching it, you get 1 pip towards ranking up, like they did in the picture, hence "Rank Progress +1".

The red diamond is a double pip. For reaching it, you'll get 2 pips towards ranking up. This generally means you're performing above expectations for your current ranking. (Not that the rankings really mean anything anymore, since there's MMR now.)

If you hover over your rank in the top right of the screen, you can see how many pips you have and how many are left before you rank up. Essentially, you just collect these pips and at some point you'll rank up.

Glad I could help.