This is an important word and nobody has it right yet. What it means is: Trying too hard, unsubtle, and inauthentic.
Specifically that which is unsubtle or inauthentic in its way of trying to elicit a certain response from a viewer, listener, audience, etc. Celine Dion is cheesy because her lyrics, timbre, key changes, and swelling orchestral accompaniment telegraph 'i want you to be moved' instead of moving you. Gold chains on an exposed hairy chest are cheesy because they shout out: "I have money and I am manly" instead of impressing a woman in a more subtle way, or allowing a woman to form her own judgments. The excessive showing off suggests he's compensating for what he does not have--i.e., he's actually poor, insecure, or short with an inferiority complex. Cliches are often cheesy because they are an obvious and artless way of making a point. A movie might be cheesy if it contains 'on the nose' dialogue, like "I can't live without you" or "You had me at hello."
Cheesiness is subjective. What seems cheesy to me, may be a legitimate and attractive hairstyle to you. What seems cheesy to me, may cause you to weep and hug your girlfriend tight.