You can read more about WebGL in the manual Technology
section and this blog post on HTML5 performance
. Enabling WebGL can help your game run 2-3x faster so it's definitely a good idea to have it on.
No mobile browsers currently support WebGL which is a shame. So it won't work with PhoneGap. As for Apple, they cite security concerns as a reason not to support WebGL, but really I think it's because it could allow HTML5 apps to compete with native apps in their app store, which they make money from.