Ace to five and ace to six are terms used to suggest that flushes and straights don't count if the game is being played for low. Since the straights and flushes can't count, it means that the ace can be played as a low card. The best hand possible in a game of poker that is an ace to five is referred to as a wheel. They are also known as two different ways of assessing low hands in poker. The wheel as it is called by the poker players is when all five cards are labelled in any order as a collection of A, 2, 3, 4 and 5.
Whether the term used is ace to five or ace to six, they are both taken to mean almost the same thing except for the fact that their flushes and straights can't be relevant to the game at that time. This implies that a low card will have to take their place for the game to move on. The two terms are only slightly different with regards to how they are being evaluated during the course of playing poker.