A reload is the periodic restarting, or reboot, of the Matrix to temporarily restore it to its full operating capacity.
After approximately 100 years of operation, the Matrix's performance and stability degrades dramatically and leaves the virtual world at risk of a catastrophic system crash that would kill all inhabitants within the Matrix, blue pill, and program alike. To counter this, the Matrix must be reloaded.
The Architect, the creator of the Matrix (and, apparently, one of its system administrators), explains this point to Neo in their only encounter. The Architect tells Neo that the programming that enabled choice for the Matrix inhabitants (created by Oracle) is also the cause of the virtual world's slow destabilization.
The Architect also explains a complication to reloading the Matrix: for some unknown reason, the Prime Program, a key programming necessary to initiate the reload, resides within the programming of the One himself. As a result, the Architect and the Oracle use the Prophecy to lure the One to the Architect's realm, whereby the Architect essentially blackmails the One into entering the door leading to the Source. If the One chooses to enter the second door, which leads back to the Matrix, the Architect warned that humanity would become extinct with the eventual crash of the Matrix and the Machines' deliberate destruction of Zion.
The current version of the Matrix itself has apparently been reset a total of 6 times since its creation. Presumably, its inhabitants had their memories erased each time it was reloaded, as well as when it was first created, which had ultimately been 7 times.
- This information is corroborated by a scene in The Matrix Reloaded where Morpheus tells the war's history to the populace of Zion on the eve of the assault by thousands of sentinels on the city. He states that the war has been waged for nearly 100 years.
- The Architect tells this to Neo during their discussion in The Matrix Reloaded.