Sebaceous glands attached to the hair follicles secrete sebum. This is a natural blend of lipids that lubricates the hair and protects it against external aggressions.
If these sebaceous glands are unbalanced and secrete too much sebum (hyperseborrhea), then the hair becomes oily. This excessive sebum covers the scalp and spreads down the hair shaft.
Visually, the hair looks almost wet.