Most engines that have hydraulically actuated self-adjusting valve actuators and do not need adjustment. The actuators on your engine are mechanical and need to be checked at set at regular (15,000 miles or so) intervals.

As the valve and adjuster wear over time, the adjuster clearance gets smaller. Beyond this point when there is no more clearance, further wear means that the valve cannot close completely and will begin to leak. When this happens you lose compression, and the engine may not run as well and can be harder to start.

There are a LOT of threads on checking and adjusting valves. Try a Search, and let us know.

Best Regards,
Jim