In sports betting, "even money" refers to when there is a bet between two sides that doesn't involve any vigorish or "Vig".
For instance - let's say that you and a buddy are betting on the outcome of a Super Bowl game. You both determine that the game is a virtual toss-up, so you decide to make an "even money" bet on the outcome of the game.
If you win, your buddy will give you $100. If your buddy wins, you will cough up $100. This is an "even money" bet.
Now - let's say that you wanted to bet on the same game at a sports book, and the game was a coin flip.
Instead of betting $100 to win $100 back, you would almost certainly have to bet $110 in order to have a chance of winning $100 back. The $10 is the "vigorish", and this is the way that sports books make their money.