A tot is a small child, like the tots at your little sister's nursery school. Tater tots, or little bitty potato puffs, are another kind of tot.
The origin of the noun tot is a little unclear, although we know it first appeared in the 18th century. It was also common in the early 20th century to refer to children's playgrounds as tot lots. As a verb, tot is an old-fashioned way to say "add," a shortened form of total. So your waiter might say, "I'll be right back with your check as soon as I tot everything up!"