Traditionally, companies spend time and money managing a server's computing resources. These represent a fixed and recurring cost to the company. It doesn't matter if the server is idle or if it is performing some form of computing task. The bottom line is that it costs money just by being there.
With serverless computing, the computing resources are scalable cloud services. This means that it is an event-driven application design. Basically, with serverless computing, you only pay for what you use. This is true of Azure Functions.
Azure Functions are small bits of code that reside in the cloud. Your applications can simply just use these functions as needed, and you only pay for the computing power used. It does not matter if one or one million people access your application. Azure Functions will automatically scale to handle the additional load. When the usage of your application drops, the Azure Function scales back down.