A floor in a house that is too dry will develop cracks between the boards. Floor boards that have taken on too much 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, although if you floor is loose, the filler most likely will fall out. A light stain can help to conceal cracks by adding contrast withing each board, rather than emphasizing the contrast (and the gaps) between boards.
Really large cracks can have "slivers" - thin strips of wood - glued into the cracks during the refinishing process.
What can we do for you? Call us for a free home estimate. 248-627-5643