Brining is the process in which you soak the meat in a brining solution for a given time. The purpose is to tenderize the meat and help hydrate the meat before cooking. The brine surrounding the cells has a higher concentration of salt than the fluid within the cells, but the cell fluid has a higher concentration of other solutes. This leads salt ions to diffuse into the cell, whilst the solutes in the cells cannot diffuse through the cell membranes into the brine. The increased salinity of the cell fluid causes the cell to absorb water from the brine via osmosis. The salt introduced into the cell also denatures its proteins. The proteins coagulate, forming a matrix that traps water molecules and holds them during cooking. This prevents the meat from dehydrating.
The larger the piece of meat it, the longer the meat must brine.