What is ELO decay?

Decaying is my job...

ELO decay occurs in ranked when you don't play for 4 weeks from your last ranked game in that specific ladder aka solo que and premade 3s or 5s.

You lose 25 ELO every 4 weeks until you hit 1400, after which you cannot autodecay any further. Generally you only gain/lose 13 ELO each game, so it's probably in your best interest to at least play one or two every 3 weeks or so.