Floors, especially hardwood, are affected by the changes in the air that happen from season to season. Humidity, temperature, and air pressure can all affect the way wood sits. This means that wood will bend or swell a little bit during the summer. In very old homes, known for squeaks and weird sounds, this kind of swelling is what is occurring.
There are several different types of swelling. One type is called cupping. It's what happens when the center of the board bends lower than the edges, making little cups. Another type of swelling is called crowning. It's the opposite – the board bends upwards. Neither of these types of swelling is harmful, and aren't much to worry about. This subtle swelling can be prevented in the construction, but it's best not to mess too much with it during the swelling season. You can try to sand it down, but it might just make the problem flip from crowning to cupping. The best thing to do, to keep the swelling minimal, is to keep the humidity low in the house. Be sure to quickly wipe up spills. With these two things, this minimal swelling will easily cure itself. Dehumidifiers are a useful tool to reduce the moisture.
If you are installing a floor and want to prevent future swelling, you must let the wood acclimate to the environment in which it will be installed. That way, it will absorb the levels of moisture that are around it and once stabilized will remain stable. Of course, you should also install moisture barriers. Beware that moisture levels will still vary throughout the year, however. Don't be too concerned if you see gaps between planks during certain seasons, as they will likely close during other seasons.
Be sure to consult whoever is installing your floor on the best moisture barriers to use, time of year to install floors, and how long you should allow the wood to acclimate before installing.
The biggest concern is if you see bucking. Bucking may require replacing floor board. It's extreme warping and involves parts of the board lifting entirely off the subfloor. Don't worry though, it is likely only going to be caused by flooding or other extreme moisture situations.