In computers, a lossy way of making a file smaller throws some of the information away on purpose. A lossy music file (many MP3s) does not keep every tiny sound; a lossy photo (many JPEGs) does not keep every tiny bit of the picture. You often cannot hear or see what was cut, but if you squeeze too hard the song or the picture starts to look or sound wrong. The opposite is lossless: the file gets smaller, but you can get every bit back. Tech people say this all the time; many ordinary speakers do not. Though actually, people sometimes use “lossy” as if it simply meant “bad quality,” but a careful lossy file can still sound or look fine.