Docker like I’m five

Imagine you have a big bucket of Lego pieces and want to build a castle. But instead of sorting through all the different components and figuring out which ones you need for the castle, you have a special Lego box with all the pieces you need, organized and ready to go.
Docker is like that special Lego box for computer programs. But, instead of sorting through all the different code pieces you need to build your program, Docker gives you a pre-built box with all the code pieces already organized and ready to use.
Like how you can take your Lego castle apart and rebuild it into something else, Docker also lets you take apart and rebuild your program differently, using the same pre-built box of code pieces.
So, Docker is like a pre-built Lego box for computer programs, making it easy to build and rebuild your program using all the right code pieces.