Boot loader is a set of program that initially runs when the machine is powered up. Boot loader is also called boot straper. In a controller like arduino boot loader enables the programming capabilities of a device.
When machine is powered up, boot loader receives information via communication cables and write the information to the controller.
In arduino, the external programmer can be used to avoid the delay of boot loader. For the burning process of boot loader the external programmer must connect to the ICSP pins of the controller