Help commit failure on trunk

I'm just playing with a sample repos with SVN.
Now I get the following when committing to trunk.
What does it mean and how do I fix it?

svn: Commit failed (details follow):
svn: Out of date: '/trunk' in transaction '25-1'

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Chris T Fouts
Qimonda NA
3000 CentreGreen Way
Cary, NC 27513
Phone: (919)677-3016
Fax: (919)678-1929
Email: chris.fouts@qimonda.com

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Received on Thu Aug 17 17:38:57 2006