The term Reboot, in media dealing with serial fiction, means to discard much or even all previous continuity in the series and start anew with fresh ideas. Effectively, all established fictive history is declared by the writer(s) to be null and void, or at least irrelevant to the new storyline, and the series starts over as if brand-new.
"Reboots" are different from "Remakes" in the sense that while a genuine "Remake" considers the series canon, along with the majority of the original, a "reboot" generally dismisses the previous releases altogether.
An example of a reboot would be the 2009 re-imagination of 1980's Friday the 13th, where the original killer "Pamela Voorhees" is entirely replaced with her son, Jason. Originally, Jason wouldn't even get a hockey mask until the third installment of the series.