Alpha-beta pruning is a technique used in game tree search to speed up minimax algorithms by pruning branches that cannot possibly change the outcome. It uses two values, alpha and beta, to track the minimum and maximum values found in the tree. If at any point it can be determined that the value of a subtree will lie outside the range between alpha and beta, that subtree can be pruned from the search. This pruning avoids searching large portions of the tree that cannot affect the result.