Per Sling, this most commonly occurs due to a temporary lapse in bandwidth available to the Sling TV application (problem with your network or problem with Sling servers), but it can also occur while video is loading after changing to a new channel. This can be due to a variety of causes, including a decrease in internet speed or a momentary breakdown of the content being delivered to your device. If this lasts for more than a couple seconds, you can try changing the channel and coming back and see if it is resolved.  If that doesn't fix it, Sling has a bunch of other troubleshooting steps, but we have found some success with changing the video quality settings.