what do we mean by negative pressure created by transpiration pull?

Hai,
Transpiration pull helps in the Movement of water through the xylem vessels (ascent).
In a leaf when water is sent out by transpiration,
a force is developed on the vessels pushing the water towards the leaf from the xylem tissue.This is called as negative pressure.
In leaf tissue after transpiration, solute particles (salts) increase causing solvent to be absorbed to nullify the concentration of solute. During this process the pressure developed in te leaf tissue is called negative pressure
The diagram attached illustrates more on the concept of negative hydrostatic pressure.