"Dropped frames" means that your connection to the server isn't stable, or you can't keep up with your set bitrate. Because of this, the program was forced to drop some of the video frames in order to compensate. If you drop too many frames, you may be
disconnected from the streaming server. Again, dropped frames are nearly impossible to be caused by OBS itself. You may also have connection problems such as random disconnections due to firewall / anti-virus / security software, routers, etc.