If you had a bet with a friend, maybe on a game of pool or some such bit of fund you would likely bet at evens.
Evens just means that you both put up the same stake and the winner takes all.
When betting on sports events with a bookmaker you will come across a whole range of odds which are meant to represent the fair chances of the outcome that you are betting in occurring.
In reality they usually represent the true chance of your selection winning plus a profit margin for the bookmaker which is called the over-round.
So if there are only two possible outcomes in an event, say a two horse race. And if both horses have equal ability and so the same chance of winning then the fair odds would indeed be evens or even money.
If you are betting with a bookmaker the odds you could get would likely be 10/11 on either horse.
So you bet £11 to get back £21 so a profit of £10 for £11 risked.
If the bookmaker takes one £11 bet on each horse then he will take £22, £11 on each horse, but only pay out £21 so making £1 on the race.
And that is essentially how bookmaking works.
But back to the original question, what does evens mean in betting, it means that if you are successful with your bet you get back double what you bet. You get your stake back and the same again in profit.