In 2015 Google said "more Google searches take place on mobile devices than on computers in 10 countries including the US and Japan". (http://adwords.blogspot.co.za/2015/05/building-for-next-moment.html) This means that chances are more people are browsing your website with a mobile device than a traditional desktop computer.
The Bootstrap grid system is mobile-first, which means it is designed to target devices with smaller displays and then grow as the display size increases. It uses a 12-column layout with different tiers for each media query range.
Think of the Bootstrap grid system as similar to a traditional HTML table. It primarily consists of three components: