It symbolizes the difference between the comforting lie and the painful truth.
That's one way of looking at it. Honestly, I think one just has to watch the movie themselves. There are varying interpretations regarding what it means.
The bare bones of it is, Tumah, that throughout the course of the film, the protagonist becomes aware that reality isn't what it seems. The lives people live are actually a complex virtual reality simulation. Taking the red pill breaks the simulation, while taking the blue pill allows one to continue existing within the simulation. How you interpret the entire thing depends very much on the ontological perspective you take to the film.