ActiveSupport provides utility classes and extensions that add functionality to Ruby on Rails. It allows checking if a variable is blank or present, accessing elements from arrays and objects by their position, performing date/time operations like checking dates from months ago, prepending or adding elements to arrays, splitting arrays into groups, and defining configurable class attributes to store global settings. Key features mentioned include blank?/present? checks, first/second for arrays, months.ago for dates, prepend for arrays, in_groups_of for splitting arrays, Configurable for global settings, and moving number helpers from ActionView to ActiveSupport.