Let's say you are going to take a four-year degree with 5 full-year courses per year. That means you'll take 20 full-year courses over the course of your four-year degree.
A major - it may be 8+ courses in one particular field like business or biology or whatever you choose. A minor - it may be 4 - 6 courses in another area. This second area can be totally unrelated to the first one.
Different schools allow different combinations of majors and minors. I completed 2 majors and 1 certificate in my four-year degree. Some people choose to do 1 major and 2 minors. I think every school will require you to have at least 1 major, and different schools will have different rules on how you can combine it with other majors, minors, or certificates.
Why have more than 1? Having a combination of majors, minors, or certificates can be useful in creating a unique skill set that may set you apart in the job market. For example, the ability to understand computer science or engineering, and biology or medicine led to the development of the interdisciplinary field of biotechnology. Some people combine business with a field that they are really interested in so they understand how businesses operate or how to start an organization that aligns with their passion. Studying more than one field also gives you a lot of flexibility in looking for job opportunities that align with your degree. It also allows you to explore more than one area of interest. In the event that you choose to switch majors, there's a chance that more of your credits may be transferable because you were already studying in that area.