What is Specificity?

Specificity determines, which CSS rule is applied by browsers.

Specificity is a type of weighting that has a bearing on how cascading style sheet (CSS) rules are displayed.
Selector specificity is a process used to determine which rules take precedence in CSS when several rules could be applied to the same element in markup.
Every selector has its specificity. All rules in CSS carry a specificity rating regardless of selector type, although the weighting that is given to each selector type varies and will ultimately affect the styling of your web documents.
If two selectors apply to the same element, the one with higher specificity wins.