A lintel is a horizontal beam placed over an opening such as a door or window to carry the weight of the wall and structure above it, transferring that load to the masonry on either side. Without a lintel, the brickwork above an opening would have no support and would collapse into the gap.
In modern construction lintels are usually RCC, cast in place or precast, with reinforcement at the bottom to resist bending. They bear a short distance onto the wall on each side of the opening (the bearing length). For wide openings or heavy loads above, the lintel may be combined with a sunshade (chajja) or designed as a deeper beam.
Traditional alternatives include stone, brick or timber lintels, still seen in older or low-cost construction. Estimating an RCC lintel is the same as any small beam: its concrete volume plus reinforcement steel from a bar bending schedule.