I'm not certain, but you might be thinking about the vascular system of plants. You probably already know that plants take in water and nutrients through their roots. In order to get that water and nutrients all the way up the stem and to all the leaves of a plant to help it grow, the plant has tissue called "xylem". Xylem looks a bit like straws that you drink through and works in a similar way, too. When water evaporates from plant leaves, the evaporation pulls water up the xylem, the same way milk gets pulled up through a straw when you suck on it. Maybe you have heard that plants take in carbon dioxide (CO₂) and create oxygen (O₂) through a process called "photosynthesis". In order to grow, plants make their own sugars out of carbon dioxide that they get from the air and water that they take up through their roots. The sugars are sometimes called "sap" and the plants have a way of moving the sap from the leaves, through the stems and into the roots. The straw-like tissues that move the sap through plants are called "phloem".

I'm guessing that it refers to the force that pulls water up through the cells of trees. This is surface tension and adhesion, weak electrostatic (i.e. chemical) interactions between water molecules and the cellulose walls of the water-transporting cells. It's the same process that occurs when you hold a sponge or paper towel partially underwater and partially out of water: the water will soak up above the water level.