what does the -i flag do in docker?

Question

I was just reading THIS article on Docker , Its an article describing how to dockerize a simple application. The following command is executed:

$ docker run -t -i ubuntu:14.04 /bin/bash,  

and then, The following explanation is given:

Here we’ve again specified the docker run command and launched an ubuntu:14.04 image. But we’ve also passed in two flags: -t and -i. The -t flag assigns a pseudo-tty or terminal inside our new container and the -i flag allows us to make an interactive connection by grabbing the standard in (STDIN) of the container.

I don't understand the meaning of:

-i flag allows us to make an interactive connection by grabbing the standard in (STDIN) 

Thank you.