What is transpiration pull theory?

It is the pulling force responsible for lifting the water column. As water is lost in form of water vapour to atmosphere from the mesophyll cells by transpiration, a negative hydrostatic pressure is created in the mesophyll cells which in turn draw water from veins of the leaves.
The negative tension is then gradually transmitted downwards via xylem tissues of the leaf, stem and finally to the roots. As a result there is a continuous upward movement of water column in the plant. 1 atm. pressure can raise water to a height of more than 32ft. So a tension of 13 atm is needed to raise water to a height of 416 feet, scientist have measured this tension to be more than 75 atm. in case of trees, more than 400 feet in height.
Thus the transpiration pull acts as pull from above on the-whole of water column of the plant which pushes the water column of xylem vessels of roots lowers leaves i.e. in an upward direction. This is how ascent of sap is affected in plants.
The whole process can be compared to a person (transpiration pull) pulling a bucket full of water with a steel rope (unbroken and continuous water column)