汽 is the mystical vital energy qi you often hear about, as in Qigong (氣功), but more generally it just means gas or vapor or steam (hence the 氵 water radical) on the left), included in many common words like 汽車 (car) and 天氣 (weather). In this case, 水 is water, so 汽水 is vapor water.
Ordering
杯 is cup (or mug or glass) and is the measure word for water (and other drinks), and 一 is one, so to order a cup of coffee, ask for 一杯汽水 (yāt būi ga fē in Cantonese, yī bēi qì shuǐ in Mandarin), one cup of vapor water.