What is application performance management? I know nothing about computers.

What kind of work would a company that specializes in application performance management do? If someone more knowledgeable than me could explain this to me as if I were 8 years old, that would be awesome.

For instance, Wikipedia says: “Application performance management, or APM, refers to the discipline within systems management that focuses on monitoring and managing the performance and service availability of software applications.”

I have no clue what this means.