Sometimes you want to wait for a particular circumstance before you act. The Ready action is a combat time option that lets you act using your reaction before the start of your next turn.

First, you decide what discrete perceivable circumstance will trigger your reaction. Then, you choose the action or the move (not in excess of your movement speed) that you will take in response to that trigger.

For example, you can say "If an orc enters my reach, I'll stab it with my dagger!" or "When I hear Taavok give the signal, I'll drop prone."

If the trigger occurs before the start of your next turn, you can take your reaction immediately after you perceive it. Alternatively, you can ignore the trigger (saving your reaction for another opportunity).

Readying a Feat or Spell: When you ready a feat or a spell, you perform the feat or cast the spell as normal but hold onto its effect while you wait for the trigger. Holding onto the magic of a spell or a Metamagic feat requires concentration (see Spell Durations) -- if your concentration is broken, the magic dissipates without taking effect.