Build a Tribute Page Feedback Help

Hi All! Here is my attempt at the “Build a Tribute Page” project:

I was having a hard time with the timeline, specifically centering it while maintaining a block display. When I finished, I decided to look at the original source code as a reference. I learned that Quincy used this line of code:

<div class="col-xs-12 col-sm-10 col-sm-offset-1 col-md-8 col-md-offset-2">

I have been trying to research a better understanding of the use of the columns but I don’t really understand. Is there someone with a better understanding of the Bootstrap grid system who can explain exactly what this line of code means? Much appreciated!


Here is the link to the original page just incase: