A solution is when one thing (the solvent, usually a liquid) dissolves something else (the solute, usually a solid). An example of a solution is salt water, where water is the solvent and the sodium chloride is the solute.
"Solution" is another name for "homogeneous mixture", which is just a way of referring to a mixture in which the composition is exactly the same in every place. This is typically achieved when solvent molecules bind themselves to the solute, keeping them from sticking back together.
As I mentioned at the top, solutions are usually solid solutes and liquid solvents, but there are others as well. Air is an example of a solution where many different gases are present, and there are catalysts in which hydrogen gas is dissolved into a transition metal.