The solute is always inthe numerator of the concentration units. The most commonly used chemistry concentration unit is molarity. I hope you are wondering why concentrations are necessary. There at least two reasons for creating concentration units. One reason is that when you study kinetics you learn that concentration influences reaction rates.
Higher concentrations mean faster reactions.
Another reason is that reactions depend on moles of reactants. The moles of chemicals can be measured using volumes of solutions instead of weighing out masses. Measuring volumes is very simple and cheap compared to measuring masses. Solutions of known molarity are made by weighing out a definite number of grams of solute. The solute is placed in a container that has a calibrated volume, like a volumetric flask.