Comprehensive car insurance covers damages from an "act of God," or events that are not caused by a car driving into something else. An "act of God" can include things like damage from a heavy tree branch falling on your car. Since you have no control over when or why a tree branch would fall on your car, this kind of accident would be covered under your comprehensive policy.
Comprehensive insurance is a coverage that helps pay to replace or repair your vehicle if it's stolen or damaged in an incident that's not a collision. Comprehensive typically covers damage from fire, vandalism or falling objects (like a tree or hail). If you're financing or leasing your car, your lender likely requires comprehensive coverage. If you own your vehicle outright, it's an optional coverage on your car insurance policy.