You dissolve a solid in a liquid by mixing the solid around until it seems to disappear. Once the solid is completely dissolved, the resulting solution behaves exactly like a liquid. Solids won't dissolve in any liquid, they must be soluble in them. For example, table salt is soluble in water but not in oil.

Solids that do not dissolve in a liquid but are mixed into a liquid can be removed with a filter. Filtering is an effective way to remove very fine particles of dirt from water.