Gulliver's Travels is Jonathan Swift's satirical novel about life in England in the early 18th century. Gulliver makes several journeys over the course of the novel, and in each place Swift focuses his satirical attack on a different facet of English life.
Gulliver's second voyage takes him to a place called Brobdingnag, where the people are huge—about ten times taller than Gulliver himself. The Brobdingnagians have a near utopian society in which people freely share the fruits of their labor. However, things are not so perfect for Gulliver, who must deal with the problem of being so small. One of the unexpected problems he encounters in Brobdingnag are insects—they are huge to him. At one point he is accosted by a fly. This wouldn't seem like anything to us, but to him the fly is the size of a bird. He finds it disgusting and it won't let him be.
It's up for interpretation as to what Swift was up to with the fly. Does it symbolize something, such as the idea that even in an almost perfect society some will have to suffer? Or is it merely an amusing interlude in between Swift's attacks on British society?