Lossless compression means that the compressed, smaller file can be expanded back into the original file without losing any information whatsoever. That is: take a file; compress it, and uncompress it again. If the original file is bit-for-bit identical, 100% of the time, for any given input file, then the compression scheme is lossless. No information is lost.

Unfortunately, compressing audio losslessly is hard. General-purpose compression programs like WinZip and gzip only manage about 5% on average. Even "next-generation" utilities like WinRAR and bzip2 only manage a few percent more.

There are special-purpose compressors (like flac) which were designed solely for losslessly compressing audio, but even they only manage about a 50% reduction in filesize on average. While this is enough for some, for music files to be truly portable they must be even smaller.