The moisture content of wood for burning should be as low as possible, but certainly below 25%.

This is particularly important if you are burning wood in a smoke exempt area in an approved appliance.
In this case the wood should be 20% or less. Wood with a moisture content above this will not burn cleanly or efficiently.

• Burning dry wood is environmentally friendly, whereas wet wood causes pollution.

• Burning dry wood will cost you less money, whereas wet wood reduces
  the heat available from the wood by up to 80%.

• Burning dry wood reduces the maintenance of the stove and substantially
  increases the longevity of the flue system.

• Burning dry wood gives a responsive fire with the finest and most
  dramatic flame pictures.

• A moisture meter is invaluable for establishing the water content of the
  wood that you are proposing to burn.

Maintenance

Always keep the instrument dry.
Prevent dirt or moisture bridging the measuring probes.
The measuring probes can be
unscrewed and replaced when blunt.
When the batteries fall below their operating voltage a battery symbol
will appear in the LCD display.
The batteries can be replaced by unscrewing the screw at the back of
the instrument above the pocket clip
to open the battery housing.
Ensure the batteries are fitted with
the correct polarity orientation. 

Accuracy 

Differing wood and differing temperatures of wood will give slightly different readings for similar moisture contents. If the wood is to be used for burning, these small variations will be insignificant.
Try to bring indoors one or two pieces of wood to dry as a reference with which to compare the progress of your wood stored outdoors.