A synovial joint is where two bones meet and allow movement. These joints are surrounded by a capsule that produces and encloses the fluid inside the joint. The capsule is connective tissue and contains many nerve endings. Thus, the joint capsule can be a source of pain and limitation of movement. Joint mobilisation can be active (i.e. when you produce the movement) or passive (i.e. when someone else moves a joint without your assistance).
In the clinical setting, joint mobilisation refers to the passive movement of joint at various speeds and amplitude, graded from 1 to 5, to help restore optimal movement, function and/or reduce pain. A manipulation is the grade 5 joint mobilisation that involves a quick thrust to the end range of movement of the joint. Sometimes a click can be heard.
This is not the sound of bones grinding against each other but rather gas bubbles leaking out of the joint capsule during the procedure. Your physiotherapist will discuss what type of joint mobilisation may be of most benefit for you.