Regas, or recharge your car air conditioining is a process of adding refrigerant into your car air conditioning system. So that you can have cool chilling air blowing out from the dash vents.
Typically, each car with air conditioning system needs to be recharge every 2 years as refrigerant gradually leaks out over time. Hence you will find air conditioning in your car is not as cold as it used to after a long period of time.
A proper system evacuation needs to be done before each recharge, in order to achieve the best possible result. During the evacuation, things like moisture, condensation, contamination objects or impurities will be removed from the air conditioning system. Imagine that you are having a newly installed ceiling, walls and carpet before you moving in your property, how nice will it be? They are plenty of DIY kits out there these days for re-gasing air conditioning but they are not tend to evacuate the system. Therefore, my advise is - bring your car to your local air conditioning workshop for a proper recharge. Spending a $100 every 2 years to get your air conditioning system maintained and avoid paying big bucks for repairing (yes, air conditioning parts are very expensive) in the future is worth it!
On the other hand, working on air conditioning system is one of the most dangerous task on vehicle servicing and repair. And you don't want the vapour of the refrigerant to leak into the air and the liquid to damage your skin if you are in contact.