Everyone remembers Neo choosing the red pill at the beginning of The Matrix, but this drug is much more than just a philosophical metaphor. Morpheus has freed many minds from the simulated cage that is The Matrix, and each time, he offers the subject a choice. Take the blue pill and forget the encounter ever happened - an instant return to the digital world crafted by Machines to appease the enslaved human population. But take the red pill, and become enlightened. See the world for what it truly is and unplug from the illusion to see reality for the very first time.
The Matrix's famous red pill/blue pill scene is widely considered a key moment in the film's philosophy. The dilemma Morpheus presents Neo is essentially Plato's Allegory of the Cave framed in the context of a science fiction dystopia. The humans inhabiting The Matrix are slaves who have only ever known shadows on the wall of a cave. Only by resisting the fear that truth brings can a person see reality, and only then can they return to guide their fellow slaves to freedom. But while choosing between the red and blue pills is a perfect visual representation of the concept of reality, these drugs actually do something tangible to their subject.
Just like everything else in The Matrix, the red pill is a piece of digital code. As revealed by the Wachowskis in 2001, Neo accepting this code allows the crew of the Nebuchadnezzar to home in on his body in the real world, so Neo can be safely extracted from the disposal system of the Machines' human battery. These power stations are so vast in size and number, that it would be almost impossible for Morpheus' crew to match an avatar encountered in The Matrix to their corresponding physical body in the real world without the tracker enabled by the red pill. This is referenced by Morpheus during his description of the two pills in The Matrix itself.
However, it seems the red pill does more than just serve as a location beacon. After lighting up the correct pod, the pill's "disruption" interferes with Neo's life support system, releasing him from an inert state. Taking the red pill causes a pod shutdown, which is evidently a dangerous process, as Neo enters fibrillation shortly before emerging from the pod fluid. The Wachowskis describe the red pill as a form of virus, and the effects certainly have the hallmarks of a classic computer attack - cracking open the Machines' technology to let the a third party in.
Over the past 2 decades, the meaning of The Matrix's red pill has shifted considerably in wider culture. Strangely, right-wing politicians have tried to take ownership of the term to push their own agendas, earning a quick and strong rebuttal from Lilly Wachowski, whose views couldn't be further removed from the public figures now trying to twist the red pill concept. The revelation that The Matrix was written as a transgender allegory adds a new spin to the red and blue pill scene. Rewatching Morpheus' monologue in 2020, the "a prison for your mind" line takes on a whole new meaning, and the pills themselves perhaps represent hormone therapy. The Matrix fans will always debate and discuss the symbolism behind the red pill, but at least the physical effects are easier to determine.
