what is an s- grade?

It's good. You get graded for how well you do on a champ in a certain role vs everyone else playing that champ in that role.

Ranking goes from C- (I think) to S+. S+ being the best, it's based on minions killed, ward score, kills, assists etc.

The idea of max rank being S comes from older japanese games where you were ranked on doing 'the thing', S being higher than an A.