What & why is HVAC maintenance needed on my 2006 Hyundai sonata ??

If it has a cabin air filter, that item needs to be replaced at the interval specified in the Hyundai Maintenance Schedule that should be sitting in your glove compartment. Check the maintenance schedule to see if cabin air filter replacement is specified for your car. Other than that, there is really no scheduled maintenance on car HVAC systems.

However, if you are beginning to experience less powerful cooling in hot weather, then repair may be needed, in the form of recharging the system with refrigerant. Some cars need this to be done every few years, some do not need it periodically. My Outback still has icy-cold A/C after 8 years with no attention other than changing the cabin air filter yearly. On the other hand, my brother’s Camry began needing a refrigerant recharge every year, starting…right after the 3rd year. This is something that really varies from car to car.

Edited to add:
If you want to be a bit proactive, you might want to periodically clean the condenser coil that is mounted in front of the radiator. A large accumulation of bugs can impede air flow through the condenser, thus making it less efficient. If there is sufficient room, a soft brush can be used to get rid of bug carcasses. If, as is typical, there is no room to get your hand between the grill and the condenser, a garden hose can be used. Just be sure that you do this while the unit is cold, and be careful to use only a gentle stream of water. If you live in The South, you might want to put a fiberglass bug screen in front of the condenser.

Please tell us why you are asking this question.
Are you experiencing problems, or is some charlatan telling you that you need to do expensive preventive maintenance on your HVAC system?

The owner’s manual, and the maintenance schedule that came with it, should answer these questions.

HVAC systems fail everyday because of the lack of maintenance. The system cannot clean or maintain itself.