In "Lestrygonians," Bloom considers trying to get an introduction to professor Joly so that he can imagine asking him about the astronomical idea of parallax (8.130). As a reader, our personal professor Joly is Don Gifford's annotations. What Gifford has to say is that parallax is "the apparent displacement or the difference in apparent direction of an object as seen from two different points of view." In astronomy, the term refers specifically to the perceived difference in direction of a celestial body (say, a star) when perceived from two different points in space.
To get your own personal idea of parallax, hold your right hand up in front of your face. Stick up your ring finger and then close your left eye. Without moving your finger, close your right eye and then open your left. Go back and forth, back and forth, and you'll realize that it seems like your finger is moving. As your vision switches from eye to another and back again, your finger seems to be displaced. This is a small and simple example of a parallax.
Parallax is a good guiding image for what Ulysses forces us to do as readers. In the course of the novel, we are exposed to three main points-of-view: Leopold's, Stephen's, and Molly's. But there are also a number of minor points-of-view that we get exposed to for brief periods of time: Father Conmee, Patrick Dignam Jr., Gerty MacDowell, and the narrator of "Cyclops," to name just a few.
The result of constantly moving from one point-of-view to another is that we can never get a handle on events and characters. We rush to come to an understanding or a judgment, and then we find ourselves re-assessing over and over again as we move through the novel. Characters offer markedly different and contradictory thoughts and opinions. If there's anything like objectivity in the novel, it is simply this accumulation of incompatible subjective personas.
One example is Bloom's affair, which we hear about first from Bloom, and then through the gossip of a number of men about town, and finally in Molly's own words. It looks different from each perspective, and it's impossible to come to any one judgment on Bloom. It's a constantly unfolding process of re-evaluation. Our opinions are always being displaced in the same way that your stable finger seems to be moving back and forth right before your eyes.