The terms 'lossy' and 'lossless' are often used when it comes to compression techniques. The Lossy technique doesn't restore your photos to their original state. It reduces the file size while lowering the image quality.
Lossless compression gives you the option of restoring your picture to its original state. It does not damage the quality of your image once it has been optimised. However, you will not get the smaller file size necessary to improve the site's load speed.