What is a Band Stop Filter?

A Band Stop Filter is a filter which blocks a set of frequencies in a specific band and permits frequencies above and below that range to pass through. 

Functionally, the Band Stop filter does the opposite of the Band Pass Filter. 

The series LC circuit is connected in parallel.  At the resonant frequency of the LC circuit, the reactance is minimum.  Thus a specific frequency is shorted across the input and prevented from reaching the output.

The Band Stop Filter can be made by connecting a Low Pass Filter and a High Pass Filter in parallel.