OnTriggerStay is called almost all the frames for every Collider other that is touching the trigger. The function is on the physics timer so it won't necessarily run every frame.
This message is sent to the trigger and the collider that touches the trigger. Note that trigger events are only sent if one of the colliders also has a rigidbody attached. Trigger events will be sent to disabled MonoBehaviours, to allow enabling Behaviours in response to collisions.