What it is
Sebum is a complex lipid blend secreted by sebaceous glands, mostly in the face, scalp, chest, and back.
It contains triglycerides, wax esters, squalene, and free fatty acids. It is a building block of the skin's surface lipid film, not simply 'bad oil'.
Sebum levels rise around puberty under androgen signaling, which is one reason both acne and seborrheic dermatitis often emerge or shift around that time.
Why it matters
Sebum is the substrate Cutibacterium acnes and Malassezia depend on. Sebum quantity and composition shape both acne and seborrheic dermatitis.