Don't panic it's normal. And it's not "running on your computer", maybe a container is running if it was some kind of service. It's not permanent. And it's actually running in a virtual machine in an operating system that's not your main deskop one.

Containers isolate themselves from the rest of your computer. It won't give you a virus or trash your machine or ransomware you or anything like that if you're using images off of docker hub.

docker ps will show you what containers are running.

docker image ls would show you what images are there.

docker run will fetch some container online, run a program, and maybe exit if it's not a server (e.g. some tools are available this way it's neat! zero installation tool use).

docker exec will let you run some command inside a container that is running e.g. some server like Nginx or Apache that you want to get a shell in.

docker-compose lets you set up a whole project of containers like e.g. a web server, a database, redis... and you could develop e.g. a web app that utilizes those services together.

Anyhoo there's the 5 second intro. Good luck!