The pressure exerted by the water in the pores on the soil is called a pour of water pressure. The negative and positive values are based upon the atmospheric pressure.
The soil below the groundwater table is fully saturated and hence the value off for pressure is greater than atmospheric pressure called a positive for water pressure.
The soil above the groundwater table is unsaturated and hence the pore pressure is less than the atmospheric pressure. Thus, the pore water pressure is negative.