1RM stands for 1 rep max. It's the heaviest weight that you can manage to do only 1 rep. If you can do anything more than that, then it's not your 1RM - you should up the weight and try again.
However, doing 1RM is extremely taxing and potentially dangerous since form goes down the drain when you're lifting your maximum, increasing the risk for injury. Hence, most people do not attempt their 1RM except for special conditions(competition, personal evaluation). Most people do several reps, and in order to have a level playing ground here, I've included a rather conservative 1RM calculator. It's not 100% accurate and will differ from person to person, but for the sake of having fun here, it's sufficient. If you squatted 50kg for 10 reps, simply enter 50 in the "weight lifted" and 10 in the "reps done" and hit "calculate" and it'll show you your estimated 1RM