John Gill Commentary 1 Samuel 21:2

"And David said unto Ahimelech the priest, The king hath commanded me a business, and hath said unto me, Let no man know anything of the business whereabout I send thee, and what I have commanded thee: and I have appointed the young men to such and such a place." — 1 Samuel 21:2 (ASV)

And David said to Ahimelech the priest
In reply to his question, and to account for such an appearance he made without an equipage:

the king has commanded me a business, and has said to me, let no
man know anything of the business concerning which I send you, and what I
have commanded you ;
he pretended he was upon a secret expedition, by the order of Saul, which none were to know of, no, not his own servants, and that was the reason why he came to him alone; which was a downright lie, and was aggravated by its being told only for the sake of getting a little food; and especially told to an high priest, and at the tabernacle of God, and when he was come to inquire of the Lord there; and was attended with a dreadful consequence, the slaughter of the Lord's priests there, which afterwards lay heavy on David's mind, (1 Samuel 22:22) ;

and is the very sin he is thought to refer to in (Psalms 119:28Psalms 119:29) . This shows the weakness of the best of men, when left to themselves; David who as much hated lying as any man did, fell into it himself:

and I have appointed [my] servants to such and such a place ;
to such a place, of such an one, not naming place nor person, that they might not be known; so the Targum calls it a place hidden and kept; and that David had some servants, though not now with him, who ate of the shewbread, appears from (Matthew 12:3) ; whom Jonathan might send after him, to a place agreed on and appointed between them; so that this might be true.