Complex sentence: An increase in x_i lowers the marginal payoff to all other player's choices x_j, so strategies are substitutes: if i chooses a higher x_i, other players have an incentive to pick a "lower" x_j.
Split into simpler sentences: An increase in x_i lowers the marginal payoff to all other player's choices x_j, so strategies are substitutes. That is, if i chooses a higher x_i, other players have an incentive to pick a "lower" x_j.