Complex sentence: The potential at a given position is a functional of the density, that is, given a particular density function and a point in space, one can compute a number which represents the potential of that point in space due to the specified density function.
Split into simpler sentences: The potential at a given position is a functional of the density. That is, given a particular density function and a point in space, one can compute a number which represents the potential of that point in space due to the specified density function.