外 is outside, and 賣 is sell (also the second character in siu mai 燒賣.
There are other ways to specify takeout depending on context and region, for example 拎走 (līng jáu) is literally carry to go, 帶走 (daai jáu) is bring to go, and 打包 (dá bāau, same 包 as all the bao) generally means “to wrap” or “to pack” (literally “tie up in a bag”) and is how you'd ask to bag up leftovers.
Refers to takeout (in Hong Kong, the British takeaway is the more common term) and delivery food. Now more than ever, Chinese restaurants advertise takeout and delivery options, and Asian grocery stores often have takeout dim sum.