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."