Linux Video plaayer?

My two boys are now enrolled in a K12 home based school. As the "learning coach" I have to open and view numerous videos every week. As an old guy I have never watched videos on my computer. When I try to open and view a video regardless of its origen (the online school, or any news video) all I get is a circling arrow with "download in progress" or simply
"player not working". Is there an onboard video player within Linux or do I have to down load a player app from some where on the internet? If I have to down load from the internet have you any suggestions which I might try?
Thank you for your time. Jared (cougarklauz@maui.net