The sign of the cross is the first prayer that most Catholic children learn, and we learn it very young: even a toddler can learn to make the sign of the cross and so begin to participate in family and ecclesial prayer.
Because it has both a verbal and a gestural component, it is also an early exposure to the Catholic practice of praying with our bodies as well as our words; this practice distinguishes Catholicism (and Eastern Orthodoxy) from most Protestant traditions.
The sign of the cross is almost always used to open any time of Catholic prayer, whether communal or individual; and is often used to end prayer as well.
The words are Trinitarian (“In the name of the Father, and of the Son, and of the Holy Spirit, Amen.”) while the gesture is cruciform (using one hand to touch forehead, chest, and each shoulder in turn, tracing a cross over our person). Thus, although it could appear to be merely some sort of etiquette (“this is what you do before you start to pray”), its expression of two fundamental elements of the Catholic Christian faith establishes it as a prayer in its own right.
The idiom “to bless oneself” means to make the sign of the cross.