A precompressor is a computer program, which alters file content so that a real lossless compression program will achieve better results than without precompressing. This is possible by creating such patterns that compression programs will recognize.
Compressing already compressed files
Usually, the compression rate on files that are already compressed (e.g. using ZIP) is poor because the data seems random to the compressor. However, compression can usually be improved by decompressing the ZIP file and recompressing it with a better compressor (e.g. one of the PAQ family). But to achieve lossless compression, the original ZIP file has to be restored using exactly the same version of the ZIP compressor and the same compression settings.
Using this strategy, certain filetypes that so far achieved poor compression rates can be processed.