A psalter is a book that contains the 150 Psalms. Before printing was invented, books were fabulously expensive. Most of the world was illiterate and poor, and poor men did not own books. Often only monastic communities and churches had books. The book of the Psalms was used for the daily recitation of prayers (something that continues today).
They strung 150 beads together, one for each psalm, so they could keep count. Other prayers and meditations were added to this beaded string of 15 decades, until we arrived at the rosary we have today.