They are powered garden tools used to trim weeds, small trees, and other dense vegetation not easily accessible by a standard lawn mower.
The models in the image appear to be backpack-style units, where the engine is worn on the user's back, allowing for greater maneuverability and reduced arm fatigue.
Brush cutters are generally more powerful than typical grass trimmers and can be fitted with various cutting heads, including metal blades, to tackle different types of foliage.
The engine is typically a petrol-powered motor that drives a fast-rotating cutting head via a long shaft.