Then came David to Nob to Ahimelech the priest: and Ahimelech came to meet David trembling, and said to him, Why are you alone, and no man with you?
"No common bread ... but there is holy bread"  (1 Samuel 21:4).  This is a reference to the Showbread, or the bread of the Presence as prescribed in the Mosaic Law  (Exodus 24:5-9).  The bread was a featured provision of the tabernacle and was replaced each week with hot bread.  It was holy from the fact of its being used in the tabernacle and only the priests were eligible to eat it.  David with his men did what was unlawful in taking and eating that bread  (Matthew 12:3-4).