What are dropouts?

I just started using the original Pixhawk, purchased from mRobotics. I have 5 flights on it so far, and on each flight the preview data log reports that I am getting dropouts. What are these, what do they mean, and what problem has to be solved?

Here is one flight preview that shows this.
https://logs.px4.io/plot_app?log=e21beda1-5a85-4802-9fce-367760d2975c

Fortunately these dropouts don’t seem to be affecting the flights, but it may mean something is not right.

Any help will be appreciated.

Thanks,
Joe