A compound set is the execution of two sets back-to-back, targeting the same muscle group.
An example would be ten repetitions of dumbbell flies immediately followed by ten repetitions of the barbell bench press.
Although the load will be less, using this training technique, the volume is higher.
Examples of compound sets:
2b. Barbell Hip Extensions
Horizontal Pushing (chest)
3a. Bench Press
4a. Barbell Row
4b. Chin Ups
Vertical Pushing (shoulder)
5a. Handstand holds
5b. Alternating Dumbbell Overhead Presses
There are many variations and combinations that can be used to fit your goals, ability level and equipment that you have access to.
Get creative, plan accordingly and get to work.