The solubility of a solute is the largest amount of solute that can dissolve in a certain quantity of

Solution.

(If this was a fill-in-the-blank)

Say you have a glass of salt water.

the salt that you have presumably stirred into the water is the solute

while the water that the solute has been stirred into is the solution.

solubility is just how much salt you can get to dissolve into the water completely before it can take no more.