A range of reps indicates that you are to find a weight that will make you fail (with good form) to complete reps somewhere between that rep range. So, if you were given a rep range of 8-12, you should find a weight that you can perform no less than 8 reps, but no more than 12.
This assumes that you have been working out for long enough that performing reps to failure within a rep range will not hurt you. It also assumes you know and use proper form.
GPP's own Brooke Lewis & Suz Martin podium the River Rampage Tri last weekend! Congrats Ladies!