Think of a container like a self contained box that can be configured to contain everything a program may need to run.
You can give the box to someone else, and they can use it on their computer without any issues.
So I could build a container that contains my program that hosts cat pictures and give it to you. As long as you have docker installed you can run a command "docker run container X" and it'll run.
This is not just an emissions thing. This saves the animals and fish who end up consuming the plastic.