First aid was any medical care offered by someone other than a medical professional to an injured person, sometimes with the use of a first aid kit. (DS9: "The Visitor")
In 2371, after The Doctor gave Kes information on first aid for burns to study, she expressed her interest in additionally learning about anatomy and physiology. (VOY: "Eye of the Needle")
In 2374, an injured Jadzia Dax assured Worf that she could administer her own care until he returned, saying she had taken basic first aid. (DS9: "Change of Heart")