The skull is the bone structure that supports the head of a human or other animal. After the person has died, the skull is the part that remains of the head after the soft tissue has decomposed. It is this lasting quality that makes it so very popular (among the living) as a symbol, not only of death itself, but of the life that used to reside within it, the legacy of a life that survives after death of the physical organism.

The bones of the skull protect the brain and blood vessels of the living head and face.  Therefore, the skull can be thought of as the seat of understanding or intelligence.  For instance, your parent or teacher tells you to "get it through your thick skull" that you must complete your homework before you can go out and have fun.  (Credit Merriam-Webster for the definition; example comes directly from this writer's experience.)

A skull symbol has become an icon for many shades of meaning, among them:

• Death in the literal sense, demise of the physical organism
• Mortality, the impermanence of physical life as we know it
• Death as a doorway to the unknown, or the afterlife, or oblivion, depending on your spiritual and philosophical beliefs
• Defiance of death
• Change, i.e., the death of one state of being giving rise to a new situation (as represented by the Death card in the Tarot deck)
• A philosophy of nihilism (nothingness)
• Fearlessness in the face of danger
• Antisocial (punk) culture
• Cynical or satirical culture, as in certain heinously expensive skull-themed art objects                      

• A fashion statement   
• Pirates (skull and crossbones)
• Poison (also with crossbones)
• Badass people (you know who you are)
• A tool used as a vessel in ritual magic
• Celebration of, and homage to one’s ancestors, as in the Day of the Dead holiday celebrated in Mexico and some other Latin American countries

• A musical affiliation (Grateful Dead band, the death metal genre, etc.)
• A motorcycle club
• Fear and fright, especially around the holiday of Halloween
• Joking about death to make light of it in the face of its inevitability

How am I supposed to interpret a skull symbol I see “in the field?” you might ask. As with all symbols, interpretation depends on context. And this is where critical thinking is quite useful! Although if you are literally out in a field and you see an actual human skull bone, you might want to a) call your local law enforcement or crime scene investigator; b) call your friend who happens to be an archeologist; c) snap a selfie while holding it with rubber gloves; or d) scream and run like hell!