Freshly cut timber still has the moisture inherent in all trees. Over time most of this moisture evaporates; this is part of the correct preparation of building timber. Ideally the timber should reach a point where the water content is in balance with its environment; at this point the timber will be stable.
Problems occur when the drying timber changes shape. Newly cut timber will bend and warp as it dries. This alteration in shape is often slight, but it is enough to cause a floor to be uneven, or for a door to become stuck. Timber can further warp if the environment has extremely high or low humidity. The timber will absorb or lose moisture and change shape.
Water content of timber is measured as a percentage, but the math for this is not the usual method used for percentages. The weight of the wood without any moisture is calculated and used as a reference point. The amount of moisture beyond this is expressed as a percentage. Hence a piece of wood that would be 1 kg when dry, but then weighs in at 1.2kg, would be calculated to have 20% moisture content. This is 100% wood and 20% water.
If the environment has a stable humidity, and there is no other water exposure, timber should reach an equilibrium point and cease to change shape. This is true whether the wood it 10 or 50 years old. Ideally, wood should have about 9- 14% moisture content. This will provide a stable material in average Australian conditions.
Measuring the humidity of your house is recommended if you have wooden floors. A stable humidity means a much more stable floor. If there are issues with higher or lower humidity it is often a simple matter to use air conditioning or indoor plants to alter the situation.