An hour is a unit of time — it's sixty minutes, 3,600 seconds, or the length it takes the long hand on your watch to move in a full circle.
If your math class lasts an hour, it might start at 9:30 and end at 10:30, one hour later. Hour also means "time of day." If your phone rings at two a.m., you could ask, "Why are you calling at such a late hour, Mom?" The word stems from the Old French hore, "one-twelfth of a day," with its roots in the Latin hora, "hour" or "season," and Greek hora, "the season."