what does positive and negative pressure in xylem challens mean?plz explain clearly

As various ions from the soil are actively transported into the vascular tissues of the roots, water follows (its potential gradient) and increases the pressure inside the xylem. This positive pressure is called root pressure, and can be responsible for pushing up water to small heights in the stem. Positive hydrostatic pressure gradient is a type of similar pressure build in a garden hose (a flexible tube used to convey water like in gardens, extinguishing fire etc.) 

But in plants, negative potential or pressure in the xylem channels plays a major role in water transport up a stem.

Negative hydrostatic pressure gradient is similar to the one that is found to occur in suction through a straw.