Ok, first off, "1RM" refers to One Rep Max meaning the absolute maximum amount of weight you can lift for a given exercise for just one rep. By definition, your 1RM could never be performed for more than one rep at a time.
Most training programs obviously have you performing more than one rep per set. In this case, you'll be lifting less than your 1RM. This can be expressed as a percentage of your 1RM.
You'll often find training programs requiring you to lift "X% of your 1RM". So if your bench 1RM is 200lbs, 90% of your 1RM would be 180lbs.
Hope this clears it up for you...