In Craps making a hard way bet means going for the total of the two dice on doubles. Say you wanted to go for a hard six, the only way to do it is with two threes. It is called the hard way because it is not easy to hit. There are only four hard way combinations: hard 4 (2x2), hard 6 (3x3), hard 8 (4x4), and hard 10 (5x5).
Player bets a "hard way" 4. Roll = 2x2 is a win, anything else, you lose (including 3x1, 1x3).