Melting is when a solid object turns into a liquid. Imagine you have an ice cube. When you put it in a cup on a hot day, it will start to melt. The ice cube turns into water that you can drink or use to water plants.
When things get hot, the molecules inside them start to move faster and faster. When they move really fast, they start to break apart from one another. This is what happens when something melts.
So, when ice melts, the temperature around the ice causes the molecules in the ice to move faster and faster. When the molecules move so fast that they break apart from one another, the ice turns into liquid water.
This is also true for other things that can melt, like chocolate or even metal. They each have different melting points, which means they will start to melt at different temperatures. Some things will melt really quickly, and some will take a long time.
In summary, melting is when a solid turns into a liquid because of the heat making the molecules inside it move really fast and break apart from one another.