The rotator cuff is a group of four muscles which stabilise movements of the shoulder at the glenohumeral joint.
You can remember them using the acronym SITS:
How to remember the functions of the rotator cuff muscles
Think back to the video and visualise where the muscles insert. The supraspinatus inserts superiorly on the greater tuberosity, so when it contracts, it causes the humerus to abduct. the infraspinatus inserts posteriorly and inferioly on the humerus, so when it contracts it pulls the humerus around laterally - external rotation. The teres minor inserts just below the infraspinatus and has the same action - external rotation. The subscapularis insert medially on the less tuberosity and when it contracts causes medial rotation of the humerus (internal rotation).