If I am going on a 10 mile hike and the book says the hike is 10 miles out and back, how many miles will it be total?
Out and back is the same as total. The 10 mile trail might be 5 miles out and 5 miles back, for example.
If you take a different route back, the numbers might not be the same. For example you might go out, around the base of something, up the far side, down the near side, then rejoin your original trail to come back to the trailhead. Maybe that would be 5.5 miles out (to the peak) and 4.5 miles back (from the peak.) But the total is what you are told.
It's said that way for two reasons: first, If I said it was a 5 mile trail I would need to clarify whether that's 5 miles each way for a total of 10, or a total of 5. Second, they are telling you the shape - it's either an "out and back" route or a "loop."
An out-and-back trail is a route that you take in one direction, then retrace your steps to get back to your starting point. It’s a hiking trail that takes you out to a specific spot and then turns around, bringing you back the same way you came. It’s often used for shorter hikes, with the trail distance from the beginning to an endpoint being no more than 10 miles. Out and back trails are great for hikers who don’t want to worry about navigating a complex network of trails with multiple intersections and forks, as well as for families who want to turn back if their hike takes too long.