Concentration
Concentration is a measure of how much solute is dissolved in a given amount of solvent or solution, from dilute to concentrated.
Definition
Concentration tells you how much stuff is dissolved in a liquid: a lot of solute makes a concentrated solution and only a little makes a dilute one. It is the amount of solute per amount of solvent or solution, given as mass percent (grams of solute per $100\text{ g}$ of solution), grams per liter, molarity ($M$, moles of solute per liter of solution), molality ($m$, moles per kilogram of solvent), mole fraction, or parts per million. Diluting a solution adds solvent and lowers the concentration without changing the amount of solute, and because the moles of solute are conserved, dilutions follow $M_1V_1 = M_2V_2$. Molarity is convenient for volumetric work but varies with temperature because volume expands, while molality does not.
Example
One spoon of drink mix in a big pitcher makes a weak, dilute drink, while three spoons in a small glass makes a strong, concentrated one. Dissolving $5\text{ g}$ of salt to make $100\text{ g}$ of solution gives a $5\%$ solution by mass, and doubling the water to $195\text{ g}$ makes $200\text{ g}$ of solution with the same $5\text{ g}$ of salt, so it is now $2.5\%$. To prepare $250\text{ mL}$ of $0.100\text{ M}$ $NaCl$, dissolve $0.0250\text{ mol} \times 58.44\text{ g/mol} = 1.46\text{ g}$ of salt in water and dilute to the mark, and diluting $50.0\text{ mL}$ of $6.0\text{ M}$ $HCl$ to $300\text{ mL}$ gives $M_2 = (6.0)(50.0)/300 = 1.0\text{ M}$.
Key Insight
Adding more water does not change how much drink mix is there; it only spreads the same amount thinner, so the drink gets more dilute. Concentration, not amount, decides how something behaves: a drop of concentrated acid burns, while a swimming pool with the same total acid spread through it is harmless. Reaction rates and equilibrium positions depend on concentrations rather than total amounts, which is why rate laws and equilibrium constants are written in terms of molarity, and chemists control reactions by controlling concentration.