Sequester means to keep separate or apart from others. In "high-profile" criminal prosecutions which have been highly publicized the jury is sometimes sequestered in a hotel without access to news media, the general public or their families except under supervision, in order to prevent the jury from having their opinions tainted by outside information. A juror has a duty to form an opinion in a case based solely upon the evidence presented in the courtroom.
Jurors may be sequestered for the entire duration of the trial or only during the deliberations portion of the trial. When a judge does decide to sequester a jury, it's often because there's a critical piece of inadmissible material being discussed in the press—for example, a suppressed confession. Or there may be the threat of harassment from reporters, protesters, or the defendant's criminal associates. Sequestration has often been used in syndicated-crime cases where a group, like the Mafia or the Hells Angels, may try to influence or intimidate a member of the jury.