1. The denominator of a fraction is the part below the fraction line. 2. A denominator cannot be zero because division by zero is undefined in mathematics. 3. If the denominator equals zero, the fraction does not represent a valid number. 4. For example, in the fraction $\frac{a}{b}$, if $b=0$, then $\frac{a}{0}$ is undefined. 5. This is because dividing any number by zero does not produce a finite or meaningful result. 6. Therefore, when solving equations, we always check that denominators are not zero to avoid invalid solutions.