Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include “If the cultist steps on the trapdoor, I’ll pull the lever that opens it,” and “If the goblin steps next to me, I move away.”
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell’s magic requires concentration (explained in chapter 10). If your concentration is broken, the spell dissipates without taking effect. For example, if you are concentrating on the web spell and ready magic missile, your web spell ends, and if you take damage before you release magic missile with your reaction, your concentration might be broken.
You have until the start of your next turn to use a readied action.
Source: PHB, page 193. Available in the SRD and the Basic Rules (2014).
in pf2e you also specify what’ll trigger the action, but the explanation for how the action works is way shorter
Ready [two-actions]
traits: Concentrate
Source Player Core pg. 417
You prepare to use an action that will occur outside your turn. Choose a single action or free action you can use, and designate a trigger. Your turn then ends. If the trigger you designated occurs before the start of your next turn, you can use the chosen action as a reaction (provided you still meet the requirements to use it). You can’t Ready a free action that already has a trigger.
If you have a multiple attack penalty and your readied action is an attack action, your readied attack takes the multiple attack penalty you had at the time you used Ready. This is one of the few times the multiple attack penalty applies when it’s not your turn.
source: archives of nethys (yes every pathfinder rule is in a free to use website)
Source: PHB, page 193. Available in the SRD and the Basic Rules (2014).
oh ok so its basically the same thing
in pf2e you also specify what’ll trigger the action, but the explanation for how the action works is way shorter
source: archives of nethys (yes every pathfinder rule is in a free to use website)