Steam boiler pressure refers to how much pressure is contained within a boiler and the pipes to which it's connected. Steam boilers operate more efficiently at higher pressures, but systems have limits.
Water expands significantly when it is boiled, and this expansion allows steam boilers to heat radiators without relying on pumps. At higher pressures, radiators warm up more, leading to faster warming and better efficiency. However, boilers and pipes are only rated to handle a certain amount of pressure, and release valves may be triggered if the system is strained too much. Systems are calibrated to ensure that they reach the optimal pressure level while in use.