"I can't spare you a whole rosary you know. Just a decade."
A rosary is the term given to a set of prayer beads used in a Catholic devotion*, as well as to the devotion itself (the devotion usually being referred to as the Rosary).
A decade is one complete sequence of prayers which is timed by moving a group of 10 beads on the rosary.
*a prayer routine used by Catholics which is not an official public prayer of the Church, i.e., not part of the liturgy.