Just what is a contextual class in bootstrap CSS?

For all practical purposes, it is an attachment to a class to enhance the presentation. The style property attaches to the class such as text-primary. So, you just use a dash to the class property value. For example, there are ten contextual colors.

Use the contextual classes to provide "meaning through colors":

class="text-muted" This text is muted.

"text-primary" This text is important.

"text-success" This text indicates success.

" text-info" This text represents some information.

"text-warning" This text represents a warning.

"text-danger" This text represents danger.

"text-secondary" Secondary text.

"text-dark" This text is dark grey.

"text-body" Default body color (often black).

"text-light" This text is light grey (on white background).

"text-white" This text is white (on a blue background because text-ligth and text-white blend too much in the white background to see! ).

The contextual class (what is written after the dash in a particular class) can be used for several classes. For example, the badge class can include the property bg-danger for the badge to appear as a waring in red.