Jamieson, Fausset and Brown on Judges 13:5

5. thou shalt conceive, and bear a son--This predicted child was to be a Nazarite. The mother was, therefore, for the sake of her promised offspring, required to practice the rigid abstinence of the Nazarite law (see on Nu 6:2).

he shall begin to deliver Israel out of the hand of the Philistines--a prophecy encouraging to a patriotic man; the terms of it, however, indicated that the period of deliverance was still to be distant.