Choc is a short, friendly way to say chocolate. British people use it a lot in shops and at home: a choc bar, hot choc, choc chips. It can mean the brown sweet food in general, or one chocolate, depending on the sentence. It sounds casual — fine with friends, odd in a formal report. Americans more often just say chocolate. You say it like 'chok'. Though actually, it is almost always written choc, not chock: that is a different word, a block you put by a wheel.