Well in ranked you get placed in your division (Silver III for me)
To move up a division you have to get to 100 LP(winning gets you LP, losing makes you lose some), and win a best of 3 to get to the next division
LP decay triggers when you haven't played a ranked game in ~30 days I think, that you will start to lose points if you still don't play
So like I'm at 49 LP currently, if I would become inactive, I think silver people lose ~10 LP a week after those 30 days.