There is always part of the liquid passing to the gas phase, and that gas makes a pressure – that is called vapor pressure. A liquid increases its vapor pressure as temperature rises, and the boiling point is reached when the vapor pressure of the liquid is equal to the atmospheric pressure. At a higher atmospheric pressure, the liquid will need to rise its temperature to an even higher temperature in order to reach enough vapor pressure. On the contrary, at a lower atmospheric pressure the liquid will reach the boiling point at a lower temperature.
This resource is commonly used for cooking. The pressure cooker makes food cook faster, because it works at 130°C instead of 100°C (at an atmosphere pressure of 1 atm, water can’t be at 130°C… because it is already vapor and it gets out of the pot). On the contrary, in sugar factories, they use a low pressure so that they can make water evaporate without cooking the sugar.