Complex sentence: The sugars and other dissolved solids do not freeze, but the water does, allowing a more concentrated grape must to be pressed from the frozen grapes, resulting in a smaller amount of more concentrated, very sweet wine.
Split into simpler sentences: The sugars and other dissolved solids do not freeze, but the water does, allowing for a more concentrated grape juice to develop. The grapes must be pressed from the frozen grapes, resulting in a smaller amount of more concentrated, very sweet wine.