Question

what is the rang of motion R.O.M. and what does it help to improve

Answer 1

progression
overload
flexibility
bone density
are the choses I think it is flexibility

Answer 2

You are right alli4476, its range of motion - how much a joint can move within its limit within its normal (anatomical) position and is measures in degrees. For example the knee has a range of motion of 30 degrees in full flexion (bent) to 165 degrees at full extension (straight) therefore the ROM is 165-30 = 135 degrees of motion.