LATCH evaluations assess the lower anchors and tethers of a vehicle. They assess how well a vehicle supports the mounting of a child safety seat. The Insurance Institute of Highway Safety will give a rating of Good, acceptable, poor or marginal. Keep in mind that the LATCH evaluation it not a safety rating. It simply assesses the efficacy, accessibility and ease of use of the vehicle's child safety seat installation.
There is a lot more to installing a car safety seat than just securing it in place with a seatbelt. The lower anchors and tethers allow you to install the safety seat more securely. A high LATCH rating means that it will be easy for you to anchor the vehicle. You should be able to install the car seat in minimal time. Accessibility is also measured during the LATCH rating.