It means it's been raining lately - the man with a meter is trying to sell you the services of his company - he is guaranteed to find dampness - all outside walls are damp - get him to do the same test on all the houses up the road and they will all show the same results as yours - unless there's a leaking gutter in which case the meter will show even more damp.
More scam than science....
Follow this link...
http://www.askjeff.co.uk/rising_damp.html
see what Geoff Howell thinks about it - buy his book for £17 and potentially save thousands...
http://www.askjeff.co.uk/books.html
Good luck...
Peccavi, February 2010