This one isn't anything fancy: a "revenant" is someone who has returned from the dead, either literally or figuratively. As it happens, this meaning gels perfectly with the book, as Glass is presumed dead but returns to strike vengeance down on his enemies.
In addition, Bridger is haunted by dreams of a strange, ghostly figure stalking and killing him. This is obviously meant to be Glass. What's more, Bridger gives us our titular reference after Glass finally shows up to exact his revenge. Take a look:
His guilt swelled up, churning like a paddle wheel in his stomach. He wanted desperately to flee. How do you escape something that comes from inside? The revenant, he knew, searched for him. (2.21.77)
In other words, "revenant" is basically just a fancy word for "guy I betrayed who's about to kill me." The more you know, right?