Freezing is when a liquid (like water) becomes a solid (like ice).
When it's really cold, water molecules (tiny little pieces that make up water) start to slow down and move closer together. As they get closer, they stick together and can't move around as much anymore because they're frozen in place.
Think of it like playing freeze tag. When you're playing, if someone tags you, you have to freeze and can't move. The water molecules are like the frozen player and they can't move because they're stuck together.
When you put something in the freezer, it gets really cold and can cause things to freeze. This is why ice cream, popsicles, and ice cubes are kept in the freezer. It helps keep them frozen so they don't melt and turn into a liquid again.
That's freezing in a nutshell!