What is the use of the HTML tag "source" ?

The <source> tag is used to specify multiple media resources for media elements, such as <video>, <audio>, and <picture>.
The <source> tag allows you to specify alternative video/audio/image files which the browser may choose from, based on its media type, codec support or media query.
An audio player with two source files. The browser should choose which file (if any) it has support for:
<audio controls>
 <source src="horse.ogg" type="audio/ogg">
 <source src="horse.mp3" type="audio/mpeg">
 Your browser does not support the audio element.