Reactions measures how quickly a player responds to a situation happening around him.
It has nothing to do with the player’s speed. A player can be really fast but at the same time have low reaction capacity. It’s the time in between the moment he sees where the ball is at and the moment he gets in position to receive it. Many people think this attribute only has importance over the players that are being controlled by the computer because the reactions of the players you’re controlling would depend only on your button-pressing reaction capacity, but this is not true. In several different situations, the player you’re controlling might or might not control or even reach the ball. When he receives a pass it’s not necessary to have a high reaction, he’ll be expecting to receive the ball, but when there’s a chance to score off a rebound, for example, or an unexpected turn of events some players just get hit by the ball, but the ones with high reaction have better chances to control it even in difficult situations. Suppose the opponent’s goalkeeper has just saved a powerful shot, you get a rebound, press shoot but your player doesn’t shoot, he just bluntly hits the ball, that happens because that player didn’t have enough reaction to getting both his body and foot in position properly to shoot. Reaction also influence dribbling, although it doesn’t quite make a player a better or worse dribbler, it allows your player to have the right reaction when the opponent goes for a sliding tackle, try and win the ball from you, and eventually it even gives your player the ability to jump over a tackle attempt.