A seborrheic keratosis is a common non-cancerous (benign) skin growth. People tend to get more of them as they get older.
Seborrheic keratoses are usually brown, black or light tan. The growths (lesions) look waxy or scaly and slightly raised. They appear gradually, usually on the face, neck, chest or back.
Seborrheic keratoses are harmless and not contagious. They don't need treatment, but you may decide to have them removed if they become irritated by clothing or you don't like how they look.
Cited from the Mayo Clinic & the American Academy of Dermatology