We use the definite article when the reader or listener understands which thing we're talking about because:
1. We've introduced it already. "The man rubs his head and says, 'Ouch! Where'd that bar come from?'"
2. There is only one in existence that matters. "We visited the Sistine Chapel when we were in Rome."
3. We describe exactly which one we're talking about. "Let's open the Chardonnay that Pam and Dale gave us for our anniversary."