If you know Tron (and how could you not), you will know just how random, adorable, and phenomenally inconsequential they are. And how cute they look shimmying along the grid.
My guess is that they had this piece of test animation which never really went anywhere, but which would have been a dreadful shame to let go to waste. So they simply inserted it somewhere, and to mask its complete randomness, they had one of the characters do an explanatory voiceover:
YORI. Boy, if these grid bugs get us, we’ve had it.
And that’s all you need to know. They are called grid bugs, and if they get you, you’ve had it. They never appear again.
I shall have a t-shirt made.