Solution concentration can be measured in a number of different ways, but the most commonly used is molarity (M).
Molarity is the moles of solute present in exactly 1 liter of solution
This does NOT mean that a liter of solution is present, it is just a ratio in the same way that density is. If I know that the density of something is 2.7 g/mL it does not mean that I have 1 mL or 2.7 g. In the same way if I know that the molarity of a solution is 4.5 mol/L it does not mean that I have 4.5 moles or that I have 1 L of solution.
Of course understanding the above requires you to know three terms about solutions: solute, solvent and solution.
What does it all mean?
Molarity tells how concentrated a solution is. So, as solution that is 3.4M (pronounced 3.4 molar) is more concentated than one that is 2.6M.
In addition, molarity allows us to relate the volume of a solution to the amount of solute present. In other words, if I know the molarity and the volume, I can determine the amount of solute. Or, if I know the amount of solute and the molarity I can determine the volume, and, of course, if i know the amount of solute and the volume I can determine the molarity.