WordPress widgets are content blocks that can be used to add different type of content & features(for example, list of latest posts, a slider, a interactive map, a contact form etc) in the theme.
Widget are used to control structure of theme by creating different layouts based on widget areas. Widgets can be added in pre-defined widget areas in a theme.
A widget can be added/removed from Theme customizer or Appearance => Widgets as shown below.
Adding widgets in Theme customizer
Adding Widget in Appearance => Widget :
You can see there are different widgets in left which can be dragged and dropped in right side widget area named “Primary Sidebar”.
WordPress themes now shipped with many widgets which can be used to enhance website look and make it feature rich. There are many plugins in WordPress plugin directory loaded with featured rich widgets.