In photography, or cinematography, or production, NG means 'No Good'. It is also expressed as 'not good' or 'not ok' or 'not go'. It is often called as a 'cheap shot' as well but sometimes it can hold somewhat different meanings depending on the context.

An NG shot is a shot that is not of expected quality. When the quality of a shot taken is compromised because of some mistakes or some faults, or somehow the director/photographer does not like it, it is then marked as an NG shot or simply NG.