It's the software that allows media ( sound and video ) to be played within an internet browser.
It's probably busy because you are using a browser that uses a version of flash player built into the browser. Google Chrome has a flash player built in but allows the option of a plug-in version of Adobe flash player. It may be necessary to disable one of these flash plug-ins in your browser settings to ensure only one flash player is operating at any given time.
Alternatively you may be using two different internet browsers ( Chrome and Internet Explorer say ) at the same time. Your PC won't know which flash player it needs to run, so close one of these browsers down.