An enzyme's active site is the specific region where substrate molecules bind and undergo a chemical reaction. This site is typically a pocket or groove on the enzyme's surface, shaped to accommodate particular substrates, much like a key fits into a lock. The enzyme’s unique structure enables it to catalyze biochemical reactions efficiently, lowering the activation energy required. Understanding the active site and its interaction with substrates is crucial for biotechnology and drug development, as it can lead to the design of inhibitors or activators that modulate enzyme activity.