“brush cutter” is a powered garden tool used to clear vegetation — grass, weeds, undergrowth, small shrubs or light bushes — in places where a lawn mower or hand tool would struggle.
A cordless brush cutter replaces the traditional petrol engine or a corded-electric motor with a rechargeable battery + electric motor. That gives you mobility (no cord to deploy, no need for mains supply), and avoids the noise, fumes, and maintenance associated with petrol engines.
Internally, it still uses the same general “brush-cutter” layout: a power unit (battery + motor), a long shaft, and a rotating cutting head at the far end of the shaft.