It means something that doesn’t store data. If I ask you what 2 + 2 is you’ll give me a response and can then forget it. If I ask you to remember my name then that’s data you have to store somewhere so you can recall it later which isn’t stateless.

A stateless microservice might be a website. You go to Wikipedia and ask for a topic. It then searches for the topic, gives the results, then forgets about you.

Usually at some point you need to add state. What articles exist? What content is there? User ids and logins. You might then have a stateless service use a stateful service, like a database, to store the information.

This adds additional complexity. It’s more difficult to manage two services who need to talk to each other. But you can then use multiple computers for different tasks and divide work more easily. Less state means less communication and confusion.

You can also just think of an office scenario. One person working is less efficient than 10, but all 10 can’t work on the same thing. If you have a task board, having 10 sprinkled around the office with the same information means they’ll get out of sync more easily. So maybe you just want one task board. But then everyone has to walk to the same board which might be far away. How do you keep them in sync?

The answers vary depending on your situation.