An anticline is the arch or crest of a fold in the rocks.
It is opposite of a syncline, which is the bottom of a fold. Its shape is concave downwards. And technically, it is a fold with younger rocks on the outside.
Why are anticlines important?
These are the preferred spots of petroleum prospectors, as in deep anticlines, the rock beds trap the rising hydrocarbon fluids. As a result, then oil and gas are build up in the pore spaces of the anticline. The largest of oilfields usually occur in large and gentle anticlines in thick sedimentary rocks.
Example: The Nittany Valley in central Pennsylvania.