The term 'Written-Down Value' basically means the value of an asset after it has been depreciated.
For example if you purchased your Vehicle on in the 2015 Financial Year for $25,000 and it was Depreciated at 25% that Year. The following will be the written down value:
Purchase Price: $25,000
Less Depreciation ($25,000 x 25%) = $5000
Written Down Value = $20,000