What is a meniscus? Why is it necessary to know about it when measuring liquids?

A meniscus occur due to surface tension within a liquid.

There are two types of meniscus - concave and convex

A concave meniscus occurs when the particles of liquid are more attracted to the container than to each other. (Eg. Water in Glass)

A convex meniscus happens when the particles of liquid are more attracted to each other than the particles of the container they are in. (Eg. Mercury in Glass)

In both cases, the accurate measurement is taken by reading the center of the meniscus.

Here is a visual representation on how to read the meniscus.