A mathematical constant is like a special number that mathematicians use a lot when solving problems. It doesn't mean it always stays the same, but it usually doesn't change very much. Examples of mathematical constants are pi (3.14), e (2.71828), and the golden ratio (1.618). They are important because mathematicians use them to find the answers to different questions. For example, pi is used to calculate the area and circumference of a circle, and the golden ratio is used to understand patterns in nature.