If you visit your grandmother and discover that she has collected so many clown dolls and clown paintings that her house is full of them, you may worry that she's become obsessed with clowns.
An obsession with something is an unhealthy, extreme interest in it. When someone is obsessed, they've lost control of their feelings about the object of their obsession. The adjective obsessed is often used to simply mean "very interested," but when someone is truly obsessed, their interest has become compulsive, and they've begun to lose control over it. The Latin root is obsessus, or "besieged," and when you're obsessed, your mind has been besieged by uncontrollable thoughts of something.