Early on, everyone learns that there are three basic states of matter: solid, liquid, and gas. That's not entirely correct, as there are also fun things like plasma (the fourth state of matter) and not-quite-states like supercritical fluids, but it's correct and reasonable enough for us to go on. Also, usually, we think of these states as existing in a line: solid, at a high enough temperature, becomes a liquid; liquid, at a high enough temperature, transitions to a gas. We imagine these states to exist on a temperature line like so:
One-dimensional graphs are so easy to make
But then we learn about fun substances like carbon dioxide, which, at a low enough temperature, transitions straight from a gas to a solid, and, when heated up, goes right from a solid to a gas, skipping the liquid phase entirely in a process called sublimation (it also makes some wicked fog that hugs the ground). Carbon dioxide does have a liquid state (all matter does), but it simply cannot be reached when at standard pressure. That's when we learn that matter state does not exist on a single line, but a plane, with both temperature and pressure affecting its state.
Liquid carbon dioxide lives in the hard-to-reach land of really high temperature and pressures
This isn't true just for carbon dioxide. For water, simply lowering the pressure of the liquid will eventually cause it to transition into a gas. You can even sublimate ice the same way, by lowering the pressure enough. Similarly, you can raise the pressure to turn water vapor into a liquid or solid, depending on the temperature. All these things are described in what is called a phase diagram, which shows what states of matter a substance is in at a given pressure and temperature.
Ignore all the weird forms of ice for now
You can pick a point on the diagram, and move in a straight line up to raise the pressure and see what happens, or a straight line down to lower the pressure. Left lowers temperature, right raises it. However, the most interesting bit is that, because there are three different states on a two-dimensional plane, all three states will nearly always touch at some point. That point, as you can see in the diagram above for water, is called the triple point.
The triple point is a pretty neat thing. For water, if you lower the pressure and temperature enough, you can have ice, liquid water, and water vapor all coexisting at the same time under the exact same conditions. But how and why does this work, exactly? Well, It comes down to how phase transitions work.
Liquid water does not instantly turn into water vapor (steam) at 100°C (212°F) at one atmosphere of pressure. That temperature brings it to the boiling point, but it actually needs a slight push to get it to move from liquid to gas. This amount of energy which must also be added to the liquid to kick off the phase transition is called the enthalphy of vaporization, or sometimes the latent heat of vaporization. This energy is what gets the molecules of the liquid, which are loosely fastened to each other, to finally, actually break free and spread out into a gas. Likewise, there is an amount of energy needed to knock the solid phase into a liquid, or a solid into a gas. This amount of energy needs to be removed from the matter when reaching a phase transition going the opposite direction as well. This means that you can have both liquid water and gaseous water sitting around at exactly 100°C and nothing will change—the gas will stay a gas, and the liquid, liquid. Similarly, you can have ice cubes at exactly 0°C (32°F) floating in exactly 0°C liquid water, and the ice won't melt, nor will the liquid water freeze. It's pretty neat (even cooler is a neat thing that liquid water can do, which is become a supercooled liquid, but that's something for another time).
What this means for the triple point is that all three states of matter can exist at the exact same temperature a pressure and coexist with each other without changing into each other. It's mad. At 0°C and 0.006 atmospheres of pressure, you can have a glass of water with ice floating in it and steam all around it, and nothing will condense, freeze, vaporize, or sublimate. And not just water, but nearly all forms of matter have this (it's possible that some are weird in a critical way, and may not). Even the most boring things around us, like water, have really cool secrets buried when you dig into them. The world is a fascinating place, full of things to discover, through science.