"Rebooting bootloader" refers to restarting the program responsible for starting up the operating system on a computer or mobile device. It is like restarting the brain of the device to fix any issues or load new software. Full definition
Reboot into Fastboot mode by typing adb reboot bootloader into the ADB shell. (android.gadgethacks.com)
Therefore, by using the lines from this step by step guide you can now learn how to safely and quickly reboot the bootloader mode on your Google Pixel XL. (android.gs)
Also, Fastboot can be used to erase partitions, entire system data or you can either use it to reboot the phone in normal mode or boot in Recovery Mode or reboot the bootloader once again as a refresh task. (android.gs)