The 'range-of-motion' of the knee is the range in degrees that it can move from fully bent to fully straightened.
'Range-of-Motion' is often abbreviated to 'ROM'.
A straight leg is said to be at zero (0) degrees (ie zero degrees of flexion). Full flexion is about 135-143 degrees, depending on the muscle/fat bulk of the limb. There is generally a different in ROM between men and women, with women having a slightly greater range than men.
Although a straight knee is at 0 degrees, normally, a person can actually go beyond 0 degrees, and slightly 'hyper-extend', so full extension might be -6 degrees. This is very important in rehabilitation, because the last few degrees of extension are critical for knee stability. So when you are rehabilitation you need to get a measurement of your normal knee - both in flexion and extension - and use that as the desired range for the problem knee.
The measurement of the range of motion is performed with an instrument known as a 'goniometer'. This is a circular plastic disc marked out in degrees, with two long arms - one stationary and the other able to move. The centre of the disc is placed on the side of the knee at a bony reference point, and the one arm is pointed at the ankle while the other is pointed at the hip.
Two variants of ROM are usually measured during rehabilitation - the 'active' ROM - ie the range that the patient can achieve unaided, and the 'passive' ROM which is the range that can be achieved if the physiotherapist takes the knee through its motion.
Terminology of flexion and extension