What does "Lossless" mean in The Computer?
Lossless usually means lossless compression, which is the opposite of lossy compression.
Compression means taking a piece of data and making it smaller using an algorithm so that it occupies less bytes in disk. In lossy compression, the algorithm discards some data to save space, and since the data has been discarded, it's not possible to revert the process to get the original data back. You "lose" the data forever. In lossless compression, reverting the compression process to obtain the original data is possible, however because this type of compression must be able to restore the original data perfectly, it can't discard any data, so the amount of space it can save is limited compared to lossy algorithms.
An example of lossless compression are images in the PNG format. PNG images are compressed, but when decompressed you get the exact colors of the pixels you had before compressing them. For audio, we have the FLAC format; FLAC stands for "Free Lossless Audio Codec."