All timber will contain some moisture. When initially cut the green timber will have a higher moisture content. Over time this moisture partially evaporates, till it balances with the surrounding environment. Moisture content of wood should stay as stable as the environment, usually the surrounding air, irrespective of whether the timber is 10 or 100 years old.
The water content of wood is measured as a percentage, but it does not follow the regular system used with percentage calculations. The amount of water in a piece of timber is divided by the weight the timber would have if it were completely dry. This means a piece of timber weighing 1.2 kg, one that should weigh 1 kg when completely dry, would be measured as 20% water content, i.e.: it contains 20% water and 100% timber. It is possible to have much higher measurements than this. Some fresh timber can have 120% moisture content. The method of calculation takes some getting used to, but it suits the moisture characteristics of wood.
When we refer to the seasoning of timber we mean the process of removing most of the moisture. Fresh, green timber has far too much water content. It is either dried in an oven or left to dry naturally in the open air. A moisture content of 9-14% is considered idea as this is the equilibrium timber will reach in most indoor environments, (at least in East coast Australia).
Dry wood is stronger and lighter that wood that still has higher moisture content. It also tends to be more stable, less prone to insect or fungal attacks, and is workable to a pleasing, smooth finish. It is also much easier to glue and treat with preservatives, lacquers, finishes and polish.
Unfortunately, the drying of timber tends to be a little unevenly. Areas that dry too much will shrink, and areas that absorb more water will slightly expand. This can cause the wood to alter its shape and slightly bend, twist or warp. This is a problem if the timber is already part of a floor or structure; the floor will be uneven or the structure will bend out of shape. The moisture content can vary by up to 5% within a single piece of timber.
There are methods that virtually eliminate the warping and ending of wood as it dries or absorbs moisture. One method is to cut timber lengthways, reverse one piece, and then glue it back together again. This causes the warping of one side of the wood to be cancelled out by the other. Another method is to layer several different types of timber. This composite board is 99% more stable than regular timber, and if the top layer is aesthetical attractive it can produce very pleasing flooring.
In practice the warping of modern wooden floors is minimal. Solid timber can be sanded down to remove issues, but in a stable environment the floor should last for many decades without issues.