For when you cannot receive Communion: at home, when ill, or not yet able to.
A prayer of desire for Jesus in the Eucharist, when receiving him sacramentally is not possible.
When: When sick or travelling, when not able to receive at Mass, while watching Mass from home, or any time during the day.
Why: The saints taught that a real desire for Communion brings real grace. It turns the heart toward the Eucharist between Masses.