Wood is porous, meaning it will soak up water, and hygroscopic, meaning that it's moisture content will vary depending on the level of humidity in the surrounding air.

If excess moisture accumulates within a wooden board, it can make the wood expand unevenly, causing the wood warp and crack.  In serious cases it can also cause the board to rot from within.

Timber used in all quality wooden chopping boards has been kiln dried, a process that removes as much moisture from the wood as possible. 

However, the wooden chopping board will need to be continually protected against moisture ingress throughout it's life to minimise it's internal water content.

The detail of the methods of protection, known as seasoning, are described in our post 'How To Protect Wooden Chopping Boards'.