An energy bar is a form of sustenance (food), designed and eaten by the Tau'ri and can also be used as military rations. It is standard practice for SG team and Atlantis team members to have energy bars in their backpack.
Years later they were pressured into escorting a young princess named Harmony on her rite of passage. She quickly began complaining that she was hungry and stole Sheppard's only energy bar from his pack while he left it unattended. She then blamed it on McKay. (SGA: "Harmony")
Energy bars were among the many supplies brought through the Stargate by the new Destiny expedition as they evacuated Icarus Base. Sgt. Spencer was caught stealing and hoarding them in his quarters, but given their unique situation, was let off with a warning. (SGU: "Air, Part 1", "Water")