What does "Lossy" mean in the Computer?
Lossy usually means lossy compression, which is the opposite of lossless compression.
When compressing data to save space in disk, we say that a compression algorithm is lossy if it discards some of the data in order to save more space. Because the data is discarded in the compression process, it's not possible to get it back by decompressing the data. In other words, it's lost forever.
Lossy compression in media starts by discarding data that is imperceptible to humans, so humans can't really tell that data has been lost. For example, in images, JPG compression removes color data from pixels, but it usually removes red and blue colors more than green, because human sight is more sensitive to small changes in green color than changes in red and blue. For audio, lossy compression such as MP3 removes data from the recorded sound waves that humans wouldn't be able to hear if those sound waves were faithfully reproduced by speakers or headphones.