What is transpirational pull?
In plants and trees, water is sucked up the stem from the roots where it evaporates at the leaves. The upward movement of the water is accounted for by a mechanism, called transpiration pull, that involves the evaporation of water from the leaves. The suction force which pulls water into the leaves (by transpiration) and up the xylem vessel is termed transpiration pull.
For a better idea of water transport in plants, watch this video:
As water evaporates from the leaves of a plant, this causes a negative pressure within the xylem (or transport cells) which in turn draws water upward from the root system and surrounding soil.