Here's the quote from the novel you need:
A strange thing happened in his head. Something flittered there in front of his mind like a bat’s wing, obscuring his idea.
At once the ideas were back, and the anger.
“We want smoke. And you go wasting your time. You roll rocks.”
As we near the end of the novel, Ralph repeatedly has these little moments of mental blackout. It's the stress of being the chief, of losing his authority, the tiredness of being on the island, and the strain of trying to manage the boys and cope with the growing fear of the beast.
He has a momentary blackout - and can't remember what he was about to say. Golding describes this blackout in the simile of a bat's wing (coloured black) literally blocking his vision.
That's the literal explanation. But figuratively, the image is more powerful: the ominous, horror-movie connotations of a bat, and its wing, coupled with the speed and suddenness of the image (think of the way a bat suddenly flitters into rapid action and imagine that jumpy shock!) creates a negative mood around the whole image. Something isn't right in Ralph's mind: he's starting to lose his clarity.
It means that Ralph is beginning, slowly, to forget his logic and sense. The same as Jack no longer cares about rescue, Ralph has started to lose his focus as chief. The bat's wing could be symbolic of the beast, which flitters (think of the way butterflies or bats fly-quickly, almost like they aren't there) into his mind for a moment, despite Ralph's efforts to block it out.