When a liquid is heated up, its vapour pressure increases with increasing temperature. The liquid boils when the vapour pressure equals the external pressure, i.e. the atmospheric pressure. If this pressure is the standard pressure of 1 atm (101.3 kPa), the temperature at which the liquid boils is referred to as its normal boiling point. When there is less atmospheric pressure, a smaller vapour pressure is required to get the water boiling, hence a lower boiling temperature.
Normal boiling point of water is 100 degrees Celsius. The atmospheric pressure at high altitudes is considerably lesser than the standard atmospheric pressure measured at sea level. Therefore, when water is heated at higher altitudes, a lower temperature is required to get it boiling. In other words, it registers a lower boiling point there.