Wood is hygroscopic. It gains or loses water moisture as the relative humidity (RH) of the surrounding air changes.
These varying humidity levels of the surrounding air cause wood to not only gain or lose water moisture but to expand or shrink as well. As the humidity increases, the MC increases, causing the wood to expand. As the humidity decreases, the MC decreases, causing the wood to shrink. When the wood neither gains nor loses moisture, we say that the wood has reached its equilibrium moisture content (EMC).