When use "Apply" and "Reset" buttons

Until you click apply, the shape in the clay tool is just a brush and you don't yet have a model. Clicking apply creates a 'clay' i.e model object, in that position. You can then continue to move the brush and add to that model (or subtract from it using the eraser..etc.

I would recommend watching a couple of videos that explain how the tools work (try the wheel video as it does show some of the tools in action and should make things clearer).

https://substance3d.adobe.com/tutorials

Dave