Painting needs paint brushes and Blender provides a Brush panel within the Toolbar when in Weight Paint Mode.
In the Data-Block menu you find predefined Brush presets. And you can create your own custom presets as needed.
The radius defines the area of influence of the brush.
This is the amount of paint to be applied per brush stroke.
- Use Falloff
When enabled, use Strength falloff for the brush. Brush Strength decays with the distance from the center of the brush.
The weight (visualized as a color) to be used by the brush.