You often see it used in medical and anatomical terms:
* Arthritis: Inflammation of a joint.
* Arthroscopy: A surgical procedure to examine the inside of a joint using a small camera and instruments.
* Arthrodesis: A surgical procedure that fuses bones together to make a joint immobile.
* Arthroplasty: A surgical procedure to replace a joint with an artificial one.
So, when you see "arthr" in a word, it usually refers to something related to joints.