This pizza is divided into $5$ slices of equal size.
Each slice is one-fifth of the pizza. We write this as $\tfrac 15$.

Someone eats $4$ of the $5$ slices.
To denote $4$ out of $5$, we write $\tfrac45$.

The number $\require{color} \definecolor{blue}{RGB}{45, 112, 179}\tfrac{\orange{4}}{\blue{5}}$ is called a fraction.
In the fraction $\tfrac{\orange{4}}{\blue{5}}$, $\orange{4}$ is called the numerator.
In the fraction $\tfrac{\orange{4}}{\blue{5}}$, $\blue{5}$ is called the denominator.

The numerator and the denominator are separated by a line, which does not have a special name in English.