The text is in Latin, and reads:

Ave verum corpus, natum
De Maria Virgine,
Vere passum, immolatum
In cruce pro homine,
Cujus latus perforatum
Unda fluxit et sanguine,
Esto nobis praegustatum
In mortis examine. 

A translation into English is:

Hail, true body,
Born of the Virgin Mary,
Truly suffered, sacrificed
On the Cross for mankind,
Whose pierced side
Flowed with water and blood,
Be for us a foretaste
In the trial of death.