Liquids freeze because molecules or atoms in the liquid are attracted to each other. Heat, or energy, acts to vibrate or move these particles, and if there's enough movement, it can overcome the attraction between these particles and they move apart. In a solid, the atoms are generally locked into place, in a specific position. However, as something heats up, the heat can provide enough energy to cause these particles to move around, and the solid becomes a liquid. Even as a liquid, the particles are bumping into each other a lot, and still have some attraction to each other. However, if you have even more heat or energy, the liquid can become a gas, where the particles barely see each other by comparison.

You can also imagine the opposite, where you have a gas, and the molecules have so much energy that if they bump into each other they fly apart violently. However, as you decrease the energy (or cool down the gas), at some point the attractive forces are strong enough to cause the particles to stick when they collide. After a certain number of these collisions, you start to get large numbers of these particles stuck together, which is a liquid. These particles are then moving and bustling around, and if you cool them even further, they lock into place because they don't have enough energy to move around, and the liquid becomes a solid.

These attractive forces can be strong or weak, and that's what determines _when_ things freeze. If the attractive forces are really strong, then something will tend to be solid at higher temperature, as the forces hold the particles together really strongly, and you need more energy to separate them.