MB is the short written form of megabyte or megabytes. A byte is a tiny unit of computer memory, and a megabyte is a large pile of them — about a million bytes. We use MB to talk about how big a file is, how much space a photo or a song takes, or how much memory a small device has. You will almost always see it in writing (3 MB, 256 MB). Out loud, people say the letters “M-B” or just “megabyte.” Be careful: Mb with a small b often means megabit, which is used for internet speed and is much smaller than a megabyte. That small change in the letter is a big change in meaning.