Essence is whatever most sums up the heart and soul of something, its truest most indispensable qualities. The essence of Las Vegas is poker chips and dreams; the essence of Johnny Cash is black clothing and country music.
Essence can be a physical detail, or, just as commonly, the abstract idea or meaning of something. The essence of Lady Chatterly's Lover might be its revolutionary eroticism, or the essence of religion its devotion to God. Essence can also mean the concentrate obtained from a flower or plant for flavoring. If you've chewed strawberry-flavored bubblegum you're probably amazed at how it truly is the very essence of strawberries.