Docker is basically a virtual machine image you write your software in. Then when you run the software you don’t need to worry about compatibility or having the right dependencies installed, it’s all included in the docker image.
Think of Docker as being Nintendo cartridges that you can take to any friends house, plug them in, and play. Servers can run more than one Docker container.
The approach greatly simplifies writing code and having it work on your server, reduces errors, and adds a layer of security.