Elo is the old way to rank players, say 1200 = scrub, 1800 = plat, etc. Now you're placed into Tiers and Divisions. But match making doesn't take that into account. You're matched up with people based on your MMR (match making record, hidden). Usually you will get match with people around your tier. You can be silver with high MMR (smurfs) and get matched up against Platinum players (with bad MMR, account buyers or boosted). Pick order is also determine by MMR. I've been first pick in rank games as gold 1 while people below me were all in plat.