The downward or upward curve at the surface of liquid in a container is said to be the meniscus of the liquid, which occurs due to the surface tension. This curve can be concave or convex depending on how the molecules of liquid interact with surface of the container. The reading of the meniscus should always be done at the eye-level.
When the particles of the liquid are more attracted to the container than to each other then a concave meniscus occurs whereas when the particles of the liquid are more attracted to each other than to the container then a convex meniscus occurs.
The bottom of the curve will be read for a concave meniscus and for convex meniscus, the top of the curve gives the correct reading of the volume of liquid.