As the name implies, this is a weld that it brushed on. It is used by the 'clever' DIY'er to 'fix' holes in cars which would otherwise fail roadworthyness test. Brush on weld takes the form of either underseal or seam sealer. Since it is much easier to apply than a proper metal weld, in that it doesnt actually join any metal together, it is within the grasp of most spanner monkeys.
Guy 1:- My car has a hole in the suspension turret and it cant pass its MOT.
Spanner monkey:- I can fix that for you using brush on weld. The MOT man will never know.
Guy 1:- You Fucking Knob Jockey! That brush on weld failed the mot!