A transposition is a known position reached by a different move order. When a certain position can be reached by two (or more) different move orders, chess players say that one variation 'is a transposition of' or 'transposes into' the other.
Although they can occur in any phase of the game, specific transpositions are seen most often in the opening. For example, the French Defense is usually reached by 1.e4 e6 2.d4 d5, but the sequence 1.d4 e6 2.e4 d5 transposes to the identical position.
Opening transpositions are in the arsenal of every good player. They are used to reach favorable positions while avoiding certain positions for the opponent. In our example of the French Defense, a player may open 1.d4 to avoid a game starting 1.e4 c5 (Sicilian Defense), where the other player is known to be a great expert.
