Lossy compression is a way to shrink a file by permanently discarding some data. In images, that tradeoff is usually acceptable because the picture still looks close to the original, even though exact pixel values are lost. JPEG is the classic example.
In cyber security, lossy compression matters because it changes what is preserved and what is destroyed. For defenders, recompressing uploaded images can strip embedded content, normalize file variants, and reduce the chance that hidden data survives inspection. For attackers, compressed media can help malicious content blend in as an ordinary image, and repeated recompression can also erase forensic detail that analysts might want to recover. Because the process is irreversible, security teams need to know when image quality, integrity, and evidence value are more important than file size.