Used on surface mount, through-hole headers and board-in connectors, a "boss" is a peg on the bottom of a header that helps polarize and situate the product onto the PCB.
For polarization, the boss allows the user to orient the product onto the PCB in only one direction. They are also used to help steady surface mount headers before subjecting them to IR reflow.
An extra hole is required to be drilled /punched into the PCB when incorporating items with bosses though the boss typically will not penetrate completely through the PCB like a through-hole solder tail would.