If you have some water in a test tube or narrow pipette, you can see that the water appears to be sticking to the sides of the tube, forming a bowl-shaped curve called the meniscus. The meniscus is formed because the adhesion of the water molecules to the glass is greater than the cohesion of the water molecules to each other.
To measure the water in the tube, you measure up until the bottom of the meniscus.
If you have mercury in a tube, like in a thermometer, the mercury forms a meniscus that bulges in the opposite direction. The mercury has greater cohesion between its own molecules than it has adhesion to the glass. Measure from the bottom of the thermometer to the top of the mercury's meniscus.