Ahh yes well...
There are two ways to compress a file, one way throws away data it deems unnecessary (ie. JPEG, simplifies the file by using an algorithm that guesses the data it threw away) this is termed lossy compression.
Then there is lossless compressed. This is what most RAW formats are, another example is FLAC or APE audio formats, they sound exactly like the source but they simply take patterns in the data and call them something simpler which the codec (decompression algorithm) knows how to return to the original state - or one indistinguishable from it.
Sony caused a controversy by creating the 12+1 lossy compressed .arw format which can occasionally cause edge artefacts because the codec is guessing how those transitions should be drawn. Most times it guesses really well.
Uncompressed is top quality
Lossless compression is also top quality but the files can take a little longer to open/process
Lossy compression is a lesser quality as it throws away some information that cannot be truly recovered.