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.
In order to read the test tube, you have to read 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: The bottom of a concave meniscus.
B: The top of a convex meniscus.
A convex meniscus occurs when the molecules have a stronger attraction to each other than to the container.
A concave meniscus occurs when the molecules of the liquid attract those of the container.
Taken from :http://education.jlab.org/qa/meniscus_01.html and http://en.wikipedia.org/wiki/Meniscus