The heroic souls are a chance to get rare toons from previous events. They typically can open to 10 different toons, ranging from 3* to 5*.
The newsfeed and book of souls fragment page let you peek at the souls available in heroics. Newsfeed also lets you look at the next two rotations. Rotations last 2 weeks (like gauntlet rotations).
Legendary Heroics are strictly the same as heroic souls, only they give out 4* or better toons from the rotation.
Mythical Heroic souls are 5* toons from the rotation.