Installing new light over sink. Has direct connection, but also has switch on light. Will not work . What is a flexible whip or nm cable

Flexible whip is a sheath with wires in it, usually metal sheath but some are not and for light leads, it uses twist wire rather than solid. You will find it on can and drop in fluorescent lights. NM is Non Metallic, commonly known as Romex. 2 or more wires plus a ground in a plastic sleeve. It is not very flexible. How was the old light wired? You may have a switch loop, what color wires on the switch?

You question does not make any sense. Are you asking about the light switch nor the cable?

NM cable is a "Non Matalic" cable. The easiest way to describe this cable is to say that outside sheath of the cable is made of a soft compound which offers little protection to the conductors inside of it.

A flexable whip, however, does have a metal outside sheath and generally comes in 6' or 8' lengths.