rEFIt is a boot menu and maintenance toolkit for EFI

Extensible Firmware Interface

The Unified Extensible Firmware Interface is a specification that defines a software interface between an operating system and platform firmware...

-based machines like the Intel Macs. It can be used to boot multiple operating systems, including triple-boot setups with Boot Camp. It also provides a way to enter and explore the EFI pre-boot environment. The name "rEFIt" is likely a play on the terms "refit" and "EFI."