A floor in a house that is too dry will develop cracks between the boards.  Floor boards that have taken on moisture will cup. A relative humidity of 30 - 50 % is recommended.  For greatest accuracy, get a digital hygrometer, or calibrate your analog hygrometer. Also, wet mopping can make floor boards cup. Wood floors should be damp mopped.

 

If your floor has permanent cracks, they can be filled with wood filler during the refinishing process. 

Really large cracks can have "slivers" - thin strips of wood - glued into the cracks during the refinishing process.