Servers are still there
A serverless app still runs on servers. The cloud provider manages them and adds capacity as more people use the app. The team supplies its code and decides what it should do.
For example, a serverless function might resize a photo when someone uploads it. It runs for that task, then stops. AWS Lambda is one service for running functions.
Useful, with limits
Serverless can save teams the work of adding servers when demand grows. Some functions have time limits or start slowly after sitting idle. Providers often charge based on use, which can make costs hard to predict.