Wood is a hygroscopic material. It takes up water from humidity and looses it to dry air.
Wood moisture depends on humidity: every change of moisture in the wood causes a change of its volume. More moisture in the wood increases its volume. Drying decreases its volume. The last case is more critical than the first: it can lead to cracks in the wood. Therefore the drying process has to be slow and controlled. At first outside, then in the workshop.
Even if you buy kiln-dried wood its advisable to let it adopt to your (hopefully dry) workshop climate. If your workshop humidity is above 65 per cent store the wood elesewhere, bring it in just for work and put it back afterwards. A small wooden closet with a 40 watts bulb left on all day long gives an ideal storage room.