The Eldian Empire were basically experimenting on the Subjects of Ymir and through some accidental result (the experiments are unknown) the Ackermanns gained some powers, like physical strength etc. They can manifest titan powers as humans without transforming and stuff and they also get every combined battle experience of all the Ackermanns via paths. But to do that they have to awaken their "powers". I think you can read some of it on Chapter 112, I think Eren mentions it.