Projection Master is a unique feature of ZBrush that allows you to use all of the 2D and 2.5D brushes in sculpting, texturing, and otherwise working with your 3D model.
The concept behind Projection Master is simple. While working with a 3D model (usually a very high-resolution one-the more "standard" 3D tools are usually used when sculpting low- and medium-resolution details), you activate Projection Master to convert the visible portion of your model into pixols on the canvas.
This simply samples the depth of your model at each visible point on the screen, and adjusts each corresponding pixol to reflect the depth, color, material, etc., of the model at that point. The model is then temporarily removed from the canvas, leaving you to work with the pixol-based representation of the model. This process is referred to as dropping the model to the screen.
Being able to use any paintbrush to affect the final geometry of the model opens up a huge number of possibilities. For example, you could use the Blur brush to smooth out a section of the model that is too rough. Or you can project other complex pieces of geometry into your mesh to create complex details as easily as a brush stroke.