First is an aggregation function available in aggregator transformation, and it returns the first value of a port (depending upon ports grouped). If no ports are grouped, Infa considers all the rows as one group and returns first value.
Syntax

FIRST(port[,filter_condition])

FIRST(pen,cost_usd > 5)

If a value is NULL, it is ignored. While if all the values are null, First returns NULL as output.

pen	cost
pentel	6
Parker	10
ABC	NULL

Output : Pentel