I have started to do a web project for school. I have made use of a template which has the following tag quite often:
You seem to be using the Bootstrap library.
<div> tag indicates the opening of a generic block-level element.
class attribute indicates that this element belongs to a named grouping. Here the classname given to it is "col-lg-12".
Bootstrap defines the style for the
col-lg-12 classname as indicating that this div should take up the entire width of the screen on large screens. Bootstrap defines the entire screen to have a size of 12 columns. And this is saying that the div will take up all 12 columns on large devices.