he means that due to the moisture content of the wood. As wood dry's out over time (particularly when brought from outside into centrally heated indoor spaces) the moisture content changes, sometimes quite rapidly. This sets up tensions in the wood and frequently causes all sorts of problems including cracking, cupping, bowing and twisting and in some cases all of the above.
With rough sawn timber, especially sleepers, they're unlikely to be kiln dried and will therefore be very very wet. Once you've assembled them, they will continue to move, ruining your design. So you need to be working with timber that has a low enough moisture content (typically below 12% for indoor projects).