Enzymes are a type of protein that catalyse reactions inside cells. They have evolved over a long time to become very good at catalysing specific reactions and are very good at chemistry, better than us mere humans!
As Daren says, an enzyme is just a catalyst in your body.
A catalyst is something that speeds up a reaction, or makes it happen at a lower energy.
Imagine to complete a reaction in your body you had to jump over a brick wall – the enzyme gives you a set of easily reachable steps so you can get over the top!