A bootloader is a small program that runs when a computer or device is powered on. Its main job is to load the operating system, such as Windows, Linux, or Android, into memory so that the device can start functioning. The bootloader checks the hardware and prepares the system for the operating system to take over. There are different types of bootloaders, including GRUB for Linux systems and U-Boot for embedded devices. Some bootloaders also allow users to choose between multiple operating systems, making it easier to run different software on the same device.