The Grand Staff is what we call it when the G Clef and the F Clef are connected together for writing down piano music.
There is a bracket, (it kind of looks like a sideways mustache) that shows that they are connected. When the clefs are connected like this, it means that they belong to the same instrument. For the piano, the G Clef tells the right hand what to play and the F clef tells the left hand what to play. The Grand Staff puts them together so both hands instructions can be put together.