Wood is an organic material that reacts to its environment. It is hygroscopic, which means that it absorbs and loses moisture in reaction to its surrounding environment. Wood gains moisture and swells in humid environments. In dry environments, it loses moisture and shrinks. This is completely normal and happens at all stages of the wood life cycle, even as the tree is growing in the forest.
If wood gains or loses too much moisture, problems can occur. Wood that gains too much moisture can cup. Cupping occurs across the width of a floor board, with edges that are raised on each board and centers that are lower than the edges. Cupping always happens due to a moisture imbalance through the thickness of the board.
Wood that loses too much moisture can gap. Gapping occurs between floor boards. Gaps can vary in size and are considered normal if they appear and disappear during seasonal changes in humidity. Gaps are not considered normal if they are large or do not close during more-humid months.
Both of these issues can be minimized by maintaining an environment that is consistently between 60-80 degrees Fahrenheit, and 30-50 percent humidity. Significant fluctuations outside these ranges can result in cupping or gaps.