Meltwater is water that comes from ice or snow that has melted. You know how when you put an ice cube in your mouth or outside in the sun, it turns into water? That's what happens with snow and ice too.
When it snows a lot or there is a lot of ice, the sun or warm weather can make it melt. This creates what we call meltwater.
Meltwater is important because it can help plants and animals, like fish, have water to drink. It also can cause rivers and lakes to get bigger because there is more water in them.
But too much meltwater can also be a bad thing. Sometimes when there is a lot of meltwater, it can cause floods or landslides. This is because the water can move really fast and can make the ground weak.
So meltwater is like when ice or snow becomes water and it can be good for plants and animals, but sometimes it can cause problems too.