Seawater is the water that you find in the ocean. It's really salty, which means it tastes different than the water you drink at home. This is because seawater has a lot of salt dissolved in it.
Think of salt like tiny little pieces of paper that are floating around in the water. There are so many pieces of paper that you can taste it when you drink the water.
Seawater is made up of different things that are in the ocean, like salt, minerals, and tiny plants and animals. These things are called "dissolved solids" because they are mixed in with the water and you can't see them.
Seawater is also really important for the planet because it helps regulate the Earth's temperature and provides a home for lots of different creatures that live in the ocean.