A header is a short beam installed horizontally over a door, window or other opening in a wall. Cutting studs out for the opening interrupts the path that roof and floor loads follow down to the foundation; the header picks those loads up and carries them sideways to the framing on each side. Each end rests on a jack stud (also called a trimmer), which is nailed to a full-height king stud.
In a 2x4 wall, the classic header is two 2x lumber pieces on edge with a 1/2” plywood or OSB spacer between them: 1-1/2” + 1/2” + 1-1/2” = 3-1/2”, exactly the wall thickness. In 2x6 walls builders often use three 2x members, or two with rigid foam between them to cut heat loss. Required size depends on the opening width, the building width, the number of floors above and the ground snow load; the IRC provides span tables in Section R602.7 — for example, a doubled 2x8 or 2x10 over a typical door in a one-story house, with larger sizes or engineered LVL headers for wide windows and garage doors.
Headers are only structural in load-bearing walls. In nonbearing interior walls the IRC allows a single flat 2x or no header at all, which is why remodelers check what a wall is carrying before cutting a new opening.