A header or sole beam in the construction and engineering world is a steel or timber beam over an opening and on the ground that disperses the structural load to the outside of the opening to keep structural integrity. Otherwise, the load from the roof, floor, or whatever is above the opening can lead to stress that can cause cracks, shifting, or even worse.
The load onto the header / sole beams has to be transferred around the opening. The header beams the load from above and the sole beam from below, and disperses it to the outside edges of the opening.
Headers & sole beams are sized based on the length of the opening and the load. We use these header and sole beams when needling and propping buildings when undertaking structural alterations.