It is kind of application which works as a button placed on your blog to view information other than your blog posts. For example, if one likes to view your profile bio, statistics of total views, your other blogs, other blogs liked by you, follow buttons and your social media sites to follow you there also, like that. All these buttons are known as widgets. All the buttons you find here on hubpages are also the same widget applications.
To remove a widget, you will click the layout or customise button in the options menu and enter into the editing mode by clicking widgets. You can add or remove widgets there. To add widget, there will be options to select from many widgets. You will select the one that suits you. There will be brief description of each widget, generally when you browse through the widgets so that you can select as per your needs.