Here's a breakdown:

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

* Substrate: The molecule that the enzyme acts upon.

* Active site: A specific three-dimensional region on the enzyme that binds to the substrate. It is typically a groove or pocket that is shaped to fit the substrate precisely.

The active site is like a lock and the substrate is like a key. The specific shape and chemical properties of the active site allow it to bind only to specific substrates. This ensures that the enzyme catalyzes the correct reaction.