To brick a device is to make it unusable, essentially turning it into the functional equivalent of an actual brick.
Strictly speaking a bricked device is broken in hardware in some way and cannot be recovered by normal means. Bricked phones, for example, normally need to be replaced or returned to the manufacturer for repair.
The term brick is also used more loosely to refer to software issues rendering a device unusable or unbootable. A bricked PC usually requires booting re-installation or recovery media in order to restore a backup image, or reinstall an operating system from scratch.