"Raise to Wake" essentially makes it so that each time you lift up your iPhone it wakes up. This makes the screen light up.
For some, this is perfect and not an issue. Others, however, noticed that sometimes your iPhone will unlock too quickly because of facial recognition. So when you lift up your iPhone, "Raise to Wake" makes it so it unlocks fully because it sees your face. And others...well, they simply don't want it to constantly light up!
Here's how to toggle "Raise to Wake" off and on in your iPhone's iOS.