If you're talking from a chemistry perspective... A "solution" is something that results from two or more substances combining to form a combined new thing.
The simplest example is salt and water. Take a spoon of salt and stir it into a glass of water. The salt is soluble in water, thus it will go into solution with the water. This essentially means the salt and water bond in a particular way so that they tend to stay together. IF, however, you spooned in some dirt and stirred that around in the water, you'd see the dirt settle out on the bottom of the glass because it does not become part of a solution with the water.