Your A C is low on refrigerant . You can have it topped off and it may last all summer, but it means it has a leak and it will happen again unless repaired. I obviously don't know how many pounds of refrigerant you unit holds or how low it is , but going price for R-22 is around $18.00 per lb. and AC's typically hold about 5 lbs plus labor cost. If you want them to locate and repair the leak the price will go up as finding a leak can take up to two hours to find plus repairing it , pulling a vacuum on system and recharging the unit so now your up to a couple of hundred . It's your decision. If this is the first time your AC has needed freon I would just top it off and see how long it lasts. If it needs it again next year then go through the whole procedure to permanently fix the problem.