lit. 'the going forth', or more fully stated, 'the going forth from home to the homeless life' of a monk (agārasmā anagāriyam pabbajjā), consists in severing all family and social ties to live the pure life of a monk, in order to realize the goal of final deliverance pointed out by the Enlightened One. Thus, p. has become the name for admission as a sāmanera, or novice, i.e. as a candidate for the Order of Bhikkhus, or monks.
See Going Fonh, by Sumana Samanera (WHEEL 27/28) - Ordination in Theravāda Buddhism (WHEEL 56).