Liquids always give off a little bit of gas, generating what's called vapor pressure. As you increase the temperature, the vapor pressure also increases. When it reaches the pressure of the atmosphere around the liquid, boiling starts to happen: the liquid can turn into a gas not just at the surface but also in the liquid itself, creating bubbles. And if you try to increase its temperature any more, you will find that you can't. It will just boil harder.

So because boiling happens when vapor pressure matches the external pressure, liquids boil at different temperatures if there are different pressures acting on it. Under high pressure, it will boil at a higher temperature; that's the principle behind a pressure cooker for example. At low pressure, it will boil at a lower temperature.