The result is a date which is a number of months away from the start date. Only months are considered; days are not used for calculation.
StartDate is a date.
Months is the number of months before (negative) or after (positive) the start date.
What date is one month prior to 2001-03-31?
=EDATE("2001-03-31";-1) returns the serial number 36950. Formatted as a date, this is 2001-02-28.