I do not know what it's officially called, but it serves the same function as bells during the period of time when bells are not allowed in the liturgy -- from just after the Gloria on Holy Thursday until the Gloria in the first Mass of Easter.
So, it would be used during the consecration on Holy Thursday, in moving the Blessed Sacrament away from the main altar after Mass on Thursday, processing the Blessed Sacrament in order to distribute Communion on Good Friday, etc.