For questions about readied actions. A readied action is an action "held" to react to an anticipated circumstance.
For questions about readied actions. A readied action is an action held in reserve for some circumstance. Depending on the combat system, it may be used to interrupt the anticipated event or only react to it.
Some examples would be to ready an action to strike an enemy as they cross the threshold of a door, to trip an enemy when they start to cross a bridge, or to attempt to grapple a spellcaster if they try to cast a spell within arms' reach.