a rep means repetition, so like 1 rep on bench press is taking the weight down and pushing it back up. you will commonly see like 3 sets of 10 reps or something like that, that means you would do 10 repetitions 3 different times with rest in between those 3 sets.
if some one hasd told you to do reps between 5-12 then they probley meant for you do 1set of 12, 1set of 10, 1set of 8 and maybe another set of 6. make sure you are going up in weight each set.
it just means any number of reps between 5-12