what exactly are participation points ?

I'm not 100% how the mechanic works, but the gist of it is that you get those points for players killed by your team, but that you did not qualify for an assist on.  You get 1/2 of a KP for each of these.  What I'm not certain of is whether there is some threshold of action that you need to meet in order to be granted these points, or if they just come for free whenever one of your team mates get kills.  I myself have not played a single game of ranked this season, nor have I watched very much ranked play at all on twitch.