Mixes InputA with InputB based on the Mask value. If Mask pixel value is 1 InputA pixel is used, if it's 0 then using InputB instead. For Mask pixel of 0.5 InputA and InputB are blended in equal shares.
The same effect could be achieved with a sequence of two Blend nodes, but sometimes it's more convenient to use Mask instead. The single Mask node is a bit faster than two Blend nodes, too.
- Invert: Inverts Mask values so that value 0 is used for InputA, and 1 for InputB