The term capilla means hair which is Latin word. A tube of very fine(hair like) bore is called capillary tube.
If a capillary tube of glass is dipped in liquid like water, the liquid rises in the tube, but when the capillary tube is dipped in a liquid like mercury, the level of the liquid falls in the tube.
This phenomenon of rise or fall of a liquid in a capillary is called capillarity.
Example: (I) We use towels for drying our skin.
(II) In trees sap rises due to vessels. It is similar to the capillary action.
We consider a capillary tube of radius r dipped in a liquid of surface tension S and density raw. Let the liquid wets the sides of the tube. Then its meniscus will be concave. The shape of the meniscus of water will be nearly spherical, if the capillary tube is of sufficiently narrow bore.
As the pressure is greater on the concave side of a liquid surface, So excess of pressure at point A just above the meniscus compared to point B just below the meniscus is
Due to this access pressure P, the liquid rises in the capillary tube to height h, When the hydrostatic pressure exerted by the liquid column becomes equal to the access pressure P.