What is f- means in digital camera?

I have Nikon n90, Many times when I want to take picture, the camera does not allow me to do anything and it show: f--
What should I do? What is wrong?

Sounds like a bad connection between the lens and the camera body.
The camera can;t seem to be able to read the aperture on your lens.

That's the "no lens" error message. Be sure that your lens is fully tightened. Sometimes new users are afraid to give the lens that final "umph" needed to make it click fully into position. Don't break anything, though! There should be a definite "click" once you have the lens fully rotated to the 12:00 position.

(Note for standard time: In spite of the time change, you do NOT rotate the lens to 11:00. 12:00 is all that is required.)