"Struck" in this case is slang for a juror not being accepted by either the prosecutor or the defense attorney. Before a trial begins, jurors are selected through a process known as "voir dire". This is where attorneys for each side gets the opportunity to ask questions of potential jurors who have been summoned for jury duty to determine if that person can be unbiased and make a fair decision regarding the guilt or innocence of the accused. The prosecutor or the defense attorney can choose not to accept a juror based on the juror's responses to her questions. That attorney then asks the judge of the trial to "strike" that juror, eliminating him/her as a potential juror. Each attorney gets to eliminate the same number of jurors without giving a specific reason, usually ten. When a potential juror is excused from jury duty, this means they have an acceptable reason, according to the courts, for not being able to serve on a jury.