Mass fill underpinning is digging down under your existing foundations to get to good ground. Ideally a test hole has been dug so you know how deep you are going to be digging, but if good ground is too deep then you are better off going for piled underpinning. Say you know good ground is a metre down from ground level such as this job in Wigan, then mass fill is the way to go. 

So sections are dug out, usually a metre wide and down to the good ground. They are then inspected before concrete is poured. Here the building inspector wanted mesh in the bottom as you can see, and the bars sticking out from both sides have been resined into the earlier excavated and concreted sections to aid with binding the separate sections of concrete together.