This is a measure of concentration in terms of moles per liter. If we stick with our lemonade analogy, it would be like measuring how many whole lemons are squeezed into each glass.
This term describes how much solute can dissolve in a solvent at a given temperature. Like figuring out how much sugar you can stir into your lemonade before it just won't dissolve anymore.
This is the process of reducing the concentration by adding more solvent (like water). If your lemonade is too strong, you'd dilute it by adding more water.