If there are size constraints, U-Boot may be split into stages: the platform would load a small SPL (Secondary Program Loader), which is a stripped-down version of U-Boot, and the SPL would do initial hardware configuration and load the larger, fully featured version of U-Boot.
It is loaded by the system's ROM or BIOS from a supported boot device, such as an SD card, SATA drive, NOR flash (e.g. U-Boot is both a first-stage and second-stage bootloader.
