What is a meniscus?
A meniscus is what happens when you put a liquid into a container. When you put water in a beaker or test tube, you see a curved surface. With most liquids, the attractive force between the liquid and the container is greater than the attraction between the individual liquid molecules. So the liquid "sticks" to the side of the container.
Your teacher might have told you that you have to read the test tube to the "bottom" of the meniscus. So, you have to hold the tube up level with your eye, and look through it to see the bottom part of the meniscus. This takes a little practice, and you have to estimate a little. But if you always use the same method, and read from the bottom of the meniscus, you will have a constant way of doing it. That will help reduce errors in your experiments.
A few liquids have a "backwards" meniscus. An example is mercury. If you put mercury in a test tube, it would be higher in the middle than at the edges.
Keith Welch, Radialogical Controls Group (Other answers by Keith Welch)