If you see For flashing when you first turn on the camera it is an error message. See page 334 of your Nikon D7100 User Manual where it appears in a list of error messages.
Problem: Memory card has not been formatted for use in camera.
Solution: Format memory card or insert new memory card
It is telling you that there is currently no memory card in the camera formatted properly for use in your camera. You need to format the memory card using the camera to format it so that the card will be properly formatted for use in your camera. You may also get this message if there are no memory cards in the camera or if only one memory card is inserted into slot #2 when there is no memory card inserted into slot #1. Page 30 of your Nikon D7100 User Manual instructs to always use slot #1 when only using one memory card in the camera.
Page 32 of your Nikon D7100 User Manual tells you how to format the card: