Every year each team is given a number of wins they're expected to meet and then people place bets on whether or not they will reach that mark. We aren't betting anything, this is only for fun and something to reference back to at the end of the year.
As an example to those who aren't familiar with this, say the Rockies are favored to win at least 65 games this season. Their over/under would be 64.5. You pick the over and you're expecting them to win more games than what they're favored, the under meaning you expect them to win less than what they're favored.
I'm making two topics, one for each league.
White Sox 90.5
Blue Jays 86.5