This <div> is in a .container. It is responsive with break points.
The default grid in Bootstrap is 12 columns wide.
Layout is based on flex.
This section is inside a row and a . Or at least I think it does.
col = column, sm = small with a breakpoint at 576px. If the browser window is less than 576px wide, the columns will collapse to a single, fluid column.
This is based on Bootstrap's Getting Started page
And check out the section on grid layout for more details.