A brick on a leash is a disparaging term for a power supply that sits in the middle of the power cord for a device. Commonly used for laptops, older flat panel monitors, some game consoles, and other devices where the engineers were unable or too lazy to design the power supply into the device itself.
Sometimes the brick only contains part of the power supply, usually the portion that reduces line voltage from 120 to 240 volts AC to a more manageable 5-30 volts DC. The rest of the power supply is in the device, typically a DC-DC converter.
As power supplies get smaller, fewer devices need these bricks as they're built into the chassis and you can use a normal IEC power cord directly.