The Value Paint tools let you modify properties of the vertices on the mesh (the points defining the shape of the model). For example, vertices can store other information such as color in addition to the positional information (X, Y, Z) they store when sculpting.
Freeze
The Freeze brush locks affected vertices so
they cannot be moved by another sculpting tool. By default, faces
appear in a blue color whenever they are frozen. To unfreeze
vertices press the Ctrl key while painting or by selecting Invert
Function in Properties before sculpting.
Mask
The Mask brush works with layers to hide and
show information stored on them. Each layer has its own mask. The
Mask brush paints the mask for whichever layer is currently
selected. To Unmask vertices, press the Ctrl key while painting or
by selecting Invert Function in Properties before sculpting.
By default, mask values are not displayed. To
view the mask select the Show Mask checkbox in the Layers window.
The mask can still be painted if it is not displayed.
Erase
The Erase brush removes sculpting from a layer.
Any sculpting that exists on the original base mesh remains
unaffected. A useful strategy is to create a layer prior to any
sculpting and paint on it so that the base layer remains unaffected
should you need to revert to the original base mesh.