Both the prosecution and the defence get to exclude ("challenge") a certain number of jurors. The idea is that you want jurors to be "generic", i. e. they shoul

Both the prosecution and the defence get to exclude ("challenge") a certain number of jurors. The idea is that you want jurors to be "generic", i. e. they should each be an average person without any experiences relevant to the case. So, for example, a recent victim of crime makes for a bad juror for the defence who will probably boot him/her of the jury.