An idiom used by military drill sergeants in an aggressive, provocative way. When a drill sergeant yells this at a new recruit at a distance of two inches, the message being conveyed is this: "Speak louder!"
Being an idiom, this is not formal, and it is far less polite - but it is not intended to be polite. It is intended to get the recruit accustomed to being yelled at and to respond in a soldier-like manner.
Even though this idiom has military roots, it is used throughout society to imply that the speaker should speak louder. Here's an example without relying on the idiom alone:
Example: Janet was trying to call Jonathan on his cell phone to tell her about Lisa's getting engaged to be married. Jonathan was having a difficult time hearing Janet's voice due to poor signal strength. He said to her, "I can't hear you! Speak louder!" Janet replied at the top of her lungs, "LISA'S GETTING MARRIED!" Jonathan replied, "Oh!! That's wonderful!"