You take the blue pill, the story ends. You wake up in your bed and believe whatever you want to believe. You take the red pill, you stay in wonderland, and I show you how deep the rabbit hole goes.
-Morpheus to Neo
The term redpill is defined as a human that is aware of the true nature of the Matrix. Redpills are typically humans whose bodies and minds have been freed from the power plant by Zion hovercraft teams, but humans still connected to the Matrix can also be aware of its reality. Usually, Redpills encounter anomalies or glitches in the system, or may free themselves from physical bonds by exceeding human limitations. In either of these cases, Other Redpills find them and offer them freedom.
The term gets its name from the color of the virtual pill given by a hovercraft operative (who has rematerialized inside the Matrix as an avatar) to a human still living out a life within the Matrix. The pill is actually a program; a tracing program that disrupts the carrier signal of the pod-human's mind, making it possible for a hovercraft operator to trace the location of the pod that holds the human, and sending commands that force the pod to go offline and awaken its inhabitant. Humans who suddenly awaken appear to be instantly rejected by docbots, who disconnect the human's body from the pod's umbilicals and eject their body to the old sewer system of the ancient human cities, where the hovercraft crew retrieves the freed human.
Of all of Zion's inhabitants, only humans who are born within the Matrix can become operatives that can reenter the Matrix, as they have the headjack that allows their minds to be retransmitted back into the Matrix's broadcast network.
Except for the Kid, all humans that have escaped the Matrix have taken a red pill. It is believed that a redpill's RSI is different than a bluepill's, enabling Agents to easily track them in sensitive areas.
A redpill is the opposite of a bluepill.