What does Piggy represent in his speech at the assembly?

I assume that you are referring to the speech which Piggy makes on the causeway leading to Castle Rock? Piggy talks to Jack's 'tribe' as if he were a teacher lecturing a class of unruly and naughty boys. This probably seems perfectly sensible to Piggy who acts, talks and thinks like an adult. But his lack of understanding of more typical boys means that he doesn't see that from the point of view of the 'tribe' they are simply being lectured to, and insulted by a fat boy, who they don't even like. The end result is that all Piggy's speech achieves is to aggravate and infuriate the boys, which is obviously not what he intended.