A blender consists of a sort of "propellor" or knife blade, driven by an electrc motor.
Ideal for transforming a lumpy mixture into something smooth and homogenized.
There are two kinds: classic blenders with a goblet into which one pours the substance to be blended, and hand blenders which are cheaper and more practical, which can be plunged directly into a bowl or pan.
If you intend to buy:
Choose the hand blender if possible, with a power rating of at least 400 Watts, with a sharp blade. It is useful to be able to take the blender apart for cleaning. On the other hand, having lots of different speeds serves no real purpose.