Depends on the ability that gives the Reaction
Some abilities will trigger before the action that triggered them, while others will trigger after the action that triggered them. Each ability should give the specifics on when this happens, if not, the effect happens immediately as per Reaction Extra:
When the triggering circumstance occurs, the effect activates
automatically, even if it is not the character’s turn.
- Interpose: This reaction happens when an adjacent ally is hit by an attack, and you can replace the target of the attack by your character. Which means that the original target was never hit and your reaction happened before the action that triggered the ability.
Once per round, when an ally within range of your normal movement is hit by an attack, you can choose to place yourself between the attacker and your ally as a reaction, making you the target of the attack instead. The attack hits you rather than your ally, and you suffer the effects normally. (...)
- Weapon Bind: After you successfully defend against a close weapon attack, your character can immediately make a disarm attempt against the attacker as a reaction. Here, the reaction happened after the action that triggered the ability.
If you take the defend action (see Defend in the Action & Adventure chapter) and successfully defend against a close weapon attack, you can make a disarm attempt against the attacker immediately as a reaction. The disarm attempt is carried out normally, including the attacker getting the opportunity to disarm you.
- Energy Aura: Here, the ability triggers as soon as someone touches your character, and if the ability triggered before the action, the attacker could die before they even actually hit your character.
You can surround your body with an aura of damaging energy or some similar effect. Anyone you touch or that touches you must make a Toughness resistance check against your aura’s Damage rank.
Which means that depending on what Trigger was set on the ability, this could happen before or after the action that triggered the ability. If you add a when I am attacked trigger to an ability, that happens before you are hit. However, if you add a when I am hit trigger to the ability, that happens after you are hit, because a hit happens after an attack roll.