Enzymes are biological catalysts that speed up biochemical reactions.
A substrate is the molecule that the enzyme acts upon and converts to product.
An active site is the part of the enzyme where the substrate binds to it. Active sites contain a specific amino acid sequence and conformation which make it easy for the substrate to bind.
Enzymes are stringently substrate specific i.e. only a certain substrate can fit in and bind to the active site of a specific enzyme, just like only one key can fit into a specific lock.
