A plinth beam is a reinforced-concrete beam constructed at plinth level — roughly where the structure meets ground level, between the foundation and the walls above. It runs around the building, tying the columns together and providing a level, continuous base on which the ground-floor walls are built.
Its main jobs are to distribute the wall load evenly onto the foundation, to tie the column bases together so they act as a unit (important during earthquakes, when it resists differential movement), and to stop cracks from propagating up from the foundation into the walls. It also acts as a barrier against moisture rising from the ground into the walls.
A typical residential plinth beam might be 230 mm wide and 230–300 mm deep, reinforced top and bottom. Estimating it is the same as any beam — concrete volume plus reinforcement — and it is an essential element in any framed (column-and-beam) structure.