If a player rolls an acey-deucey (= a 1 and a 2, also called an Ace and a Deuce), he plays the 1-and-2; then they choose any number from 1 to 6 and act as if they had just thrown a doublet of it; then that player takes another turn.
An ace can be the lowest card of a straight (ace, 2, 3, 4, 5) or the highest card of a straight (ten, jack, queen, king, ace), but a straight can't "wrap around"; a hand with queen, king, ace, 2, 3 would be worthless (unless it's a flush).