Once you reach rank 10 on your account. you get a chance each week to try some of the new heroes for free. There are 4 heroes available in total in one rotation and they are available to play in the casual modes.
When the specific heroes are in the rotation, you can finish challenges that require these heroes. After the end of the rotation, these 4 heroes will be locked again but you can unlock them at any time for War Points or Credits.
You can also unlock all current and future heroes by purchasing the Founders pack in the in-game store.