Imagine you are working on an analysis in R and you send your code to a friend. Your friend runs exactly this code on exactly the same data set but gets a slightly different result. This can have various reasons such as a different operating system, a different version of an R package, et cetera. Docker is trying to solve problems like that.
A Docker container can be seen as a computer inside your computer. The cool thing about this virtual computer is that you can send it to your friends; And when they start this computer and run your code they will get exactly the same results as you did.