Rast - ראשט/  ראסט/  ראשת-  is the first maqam in the arrangement of the Oriental maqamat and therefore represents a beginning.

Rast is the first maqam section in the vast majority of historical pizmonim collections.

The word 'Rast' sounds similar to RAS, the Arabic word for head. As a result, we use Maqam Rast in the prayers for when we begin a new Humash as well as the Shabbat Afternoon Service because that is the first time the upcoming parasha is read.