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.

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

Any help will be appreciated.