A colloquial abbreviation of 'chotto' used to get someone's attention, say 'wait', or as a casual interjection ('hey', 'hold on').
ちょ、今忙しいから後で話して。
Hey, I'm busy right now — talk to me later.
ちょ、危ない！足元を見て。
Hold on — watch out! Look at your feet.
ちょっ（と）待って、財布忘れたかも。
Wait a sec — I might have forgotten my wallet.