Eval can be used to run matlab commands which are not known in advance, but only at runtime. In your example eval executes the commando 'save test\FileName codebook' (with filename the specific filename constructed above).
The use of eval is not recommended http://www.mathworks.nl/help/matlab/matlab_prog/string-evaluation.html and in your case, it can simply be replaced by using the different syntax of the save commando ('help save') e.g.
save(sprintf('test\\%s',FileName),'codebook');
