Many devices such as routers have firmware that can be updated. If you interrupt the
update process, you won’t be able to recover because the firmware that handles
upgrading will be damaged. You will have made your router utterly useless, a brick.
Hence to brick means anything you do to firmware device to
make it unable to update it firmware. This is one reason you always update from a
local copy of the firmware software on disk, not over the Internet.