A reversible reaction does not stop; it gets BORED. Forward and reverse settle at the same speed, the concentrations stop drifting, and the mixture sits there looking still while both reactions run flat out underneath. What is constant at that point is not any one concentration — it is one particular combination of them, and that combination is $K_c$.

For the one-to-one equilibrium $\mathrm{A} + \mathrm{B} \rightleftharpoons \mathrm{C} + \mathrm{D}$, read aloud K-c equals C times D over A times B: $K_c = \dfrac{[\mathrm{C}][\mathrm{D}]}{[\mathrm{A}][\mathrm{B}]}$. Every letter has a job. Square brackets mean concentration in mol/L. $[\mathrm{C}]$ and $[\mathrm{D}]$ are the two PRODUCTS — what sits to the right of the arrow — and they go on top. $[\mathrm{A}]$ and $[\mathrm{B}]$ are the two REACTANTS, and they go underneath. $K_c$ itself carries no unit: each concentration is quietly divided by the standard state $c^{\circ} = 1\ \mathrm{mol/L}$ before it is used, which is the step nobody writes down and every table assumes. When the coefficients are not all one, they become EXPONENTS: $2\mathrm{NH_3}$ contributes $[\mathrm{NH_3}]^{2}$, never $2[\mathrm{NH_3}]$.

Two species are always thrown out of the expression: pure solids and pure liquids. Not because chemistry is squeamish, but because their concentration cannot change — a block of calcium carbonate has the same mol/L whether you have a gram of it or a quarry. A constant hidden inside a constant is still a constant, so it is absorbed into $K$ itself and never written. Gases and dissolved species stay; solids and pure liquids go.