Quite simply:

Enzyme + Substrate = Product

1. The substrate (AKA reagent) binds with the enzyme’s active site to form Enzyme-Substrate complex.
2. Enzyme reacts with substrate to produce a Product, which is then released.
3. Product released, and enzyme ready for more substrate.

Enzymes are:

• One of more polypeptide chains – so a protein – but that form a structure with an ‘Active Site’.
  
• The active site is ‘specific’, which essentially means it only has a few substrates that will fit it, and bind with it.