Capillary action is the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension.
In less technical terms, capillary action occurs because water is sticky, which means that water molecules stick to each other and to other materials. Sticking to the walls of a tube or pore will cause an upward force on the liquid at the edges and this will lead to the water moving up along the wall. Because water is sticky – water tends to stick together in a drop or similar – the upper water molecules will draw other water molecules upwards with it. This effect is so strong that it can overcome gravity. An example is if you dip a paper towel into a glass of water, the water will “climb” onto the paper towel. In fact, it will keep going up the towel until the pull of gravity is too much for it to overcome.