Widgets are tools that you can drag from the toolbar in your site's admin view into areas on your websites page. Along with your site structure and template they are the building blocks of your website.
The areas you can place a widget are highlighted as you drag the widget with a blue bar. Remember there is sometimes more than one place you can drop a widget and you can drop above or below any existing widgets.
The widgets available in your site include those listed on the left hand side.
Explore each one to find out what it is and how you can use it in your site.