When logs are first cut they usually have a moisture content around 80% depending on species, which is very high. These newly cut logs need to be dried before they can become of any use, sold or distributed. To dry them they use a kiln system where it dries large amounts of lumber.
Now here is where problems occur, depending on the wood some can have a high moisture content and others can have a lower content. These need to be seperated in order to ensure a general moisture content with all the material. If not measured some lumber may either be more moist or dry than the other lumber. These problems can go unnoticed until they are cut again creating problems such as cracked or soft wood.
For that you will need a moisture meter. A moisture meter will allow you to sort different moisture contents accordingly so that your lumber will all have very close moisture contents after being dried.