What is the capillarity? How this phenomenon chemically explained? What is relevance of the capillarity for water transport in plants?

The Capillarity is the phenomenon through which water moves inside extremely thin tubes (capillaries) aided by the attraction between the capillary wall and water molecules. The capillarity phenomenon is possible since water is a polar molecule and forms intermolecular hydrogen bonds.

Consequently there is electrical attraction (adhesion force) between the capillary wall and the water molecules that then pull each other (cohesion force) since they are bound and not just water but other liquids may move inside capillaries by capillarity.

The Capillarity isn't too relevant for the transport of water in plants. It is contributes only few centimetres of ascension.