Jamieson, Fausset and Brown on 1 Samuel 2:18

18. But Samuel ministered before the Lord, being a child--This notice of his early services in the outer courts of the tabernacle was made to pave the way for the remarkable prophecy regarding the high priest's family.

girded with a linen ephod--A small shoulder-garment or apron, used in the sacred service by the inferior priests and Levites; sometimes also by judges or eminent persons, and hence allowed to Samuel, who, though not a Levite, was devoted to God from his birth.