What is a widget? WordPress widgets are independent elements of the site. They add functionality to the sidebar, footer or other part of the website.
Widgets include, for example, the latest comments, post archive, recent posts list, category list, tag list, tag cloud, text widget, image, gallery, search form and calendar, navigation menu, HTML code, RSS and many more.
The options for adding them vary depending on the theme and plugins you use. For example, in my blog they are used in the sidebar:
Where to find WordPress widgets?
A list of available widgets can be found in the WordPress administration menu under Appearance. In the left half of the page there is a list of them, and in the right there is a list of areas of your theme where you can use them.
How do you add a widget in WordPress?
To insert the selected widget into the sidebar, grab it with the mouse and drag it to the desired location. You can also click the widget and then choose where to paste it.
After creation, a window with configuration options opens. Click Save to save the settings. If you want to delete the widget, click Delete.
You can also manage widgets thanks to live page views. You can do this by clicking the Manage with live preview button at the top of the page.
You can find more information in this official WP tutorial.