From Halopedia, the Halo wiki
A call sign is a nickname used to identify a military pilot. The use of call signs has existed for almost as long as there has been aviation, from the pilots of Earth's World Wars to modern Pelican pilots.
A pilot's call sign is used primarily in radio conversations. Though strictly unofficial, these names are adopted almost like a second name for the person they are attached to and thus, they are usually given by other pilots or coworkers. Each name varies and the reasons they are given vary as well.