The substrate that fits into an area of an enzyme is called the active site.

Here's a breakdown:

* Enzyme: A biological catalyst that speeds up chemical reactions without being consumed in the process.

* Active Site: A specific region on the enzyme's surface where the substrate binds. It has a unique shape and chemical properties that complement the substrate.

* Substrate: The molecule that the enzyme acts upon.

Think of it like a lock and key:

* Lock: The enzyme's active site

* Key: The substrate

The substrate fits perfectly into the active site, like a key fitting into a lock. This interaction allows the enzyme to catalyze the chemical reaction and transform the substrate into a product.