Minmax considers all the possible ways the game can go and returns the best value for that move, assuming the opponent also plays optimally.

A program to illustrate the Minimax Algorithm

function minimax(board, depth, isMaximizingPlayer):
 
  if current board state is a terminal state :
    return value of the board
 
  if isMaximizingPlayer :
    bestVal = -INFINITY 
    for each move in board :
      value = minimax(board, depth+1, false)
      bestVal = max( bestVal, value) 
    return bestVal
 
  else :
    bestVal = +INFINITY 
    for each move in board :
      value = minimax(board, depth+1, true)
      bestVal = min( bestVal, value) 
    return bestVal