A repetition maximum (RM) is the the most weight you can lift for a defined number of exercise movements.
A 1 RM, for example, is the heaviest weight you can lift if you give it your maximum effort. A 1RM is your personal weightlifting record for any particular exercise. It could be a bench press, squat or deadlift or any other.
A 10RM would be the heaviest weight you could lift for 10 consecutive exercise repetitions.
Depending on what you want to achieve from your sessions, Training to your RM is essential if you want to meet your goals and improve muscular strength and endurance.