When I open the Android SDK in Android Studio:
Name URL
!Glass Development Kit, Google Inc https://dl-ssl.google.com/glass/gdk/addon.xml
Now I wanna to know this "!" before the Glass Development Kit, Google Inc
,does it impact my project?
This issue was reported recently, describing the same problem that you're having, and from what's said on that thread, it looks like it was fixed already.
If you're using Android Studio 2.0 preview, that's most likely your problem.
Try to update your Android Studio and it should disappear.
By the way, it doesn't seems to be a problem at all, even if you don't update, it should work just fine.