A bootloader is a small program that gets executed at startup time. The bootloader loads data and other startup process into RAM where they can be executed.
Since RAM is a stateless memory, most computers have no way of executing programs from memory when they are first started. The bootloader's job is to take data from storage devices (such as hard drives or flash memory) and load that into RAM where it can be processed.
Ubuntu ships with the GRUB2 bootloader by default, although others can be installed.