Amylase is an enzyme that makes the process of starch breaking down into simple sugars a lot faster.
Starch is a substance found in plants. It contains sugar molecules joined chemically and is found in high levels in foods such as potatoes, rice, corn, and wheat. This is an example (if not one of the most common) of a complex sugar (called a polysaccharide in biology).
On the other hand, a simple sugar (a monosaccharide) is a sugar in its most basic from. Many of these can join together to make more complex sugars.
Complex sugars (especially carbohydrates) are one of our main sources of energy. Without enzymes, starch and other complex sugars would take a really long time to break down into simple sugars in the body, meaning that the body would not be able to absorp as much sugar, leaving us tired due to the lack of energy.