Freon is the cooling agent used in most air conditioning systems. Every air conditioning system needs a refrigerant (also called a coolant) that actually creates the cool air -- that's the role of Freon.
As necessary as Freon is to the optimal performance of your your air conditioner, you do not want to handle a potential Freon problem yourself. If you do believe your air conditioner's refrigerant lines might need recharging, call a professional to do it. In fact, it's probably a good idea to call a professional to inspect your air conditioning system just before the beginning of each cooling season. There are very few air conditioning repairs you can handle yourself, so it makes sense to bring in a professional to give your AC a check-up before you need it to do any serious cooling (in the hottest summer months).
However, one repair you might be able to make yourself is replacing the insulation around the cooling lines. You can check out the refrigerant lines to see if they look a little worn. Most refrigerant lines run from the condenser, which sits outside your air conditioning system (and probably outside your house), to the evaporator inside the house. The refrigerant lines are covered with insulation. If the insulation looks worn, your air conditioner is not working as efficiently as it can be. So you'll want to have the insulation on the lines replaced (you don't need to replace the lines themselves). Make sure you buy the exact kind of insulation that originally covered the refrigerant lines. Your air conditioner should have come with manufacturer instructions as to how to replace the refrigerant lines' insulation.
Remember, before you do any work on your air conditioner make sure you've turned off both the condenser and the evaporator.