This band rejector filter, which uses the 741 again, prevents signals of a certain frequency, in this case around 1 kHz, from passing. The rejection is basically due to the double T at the input, whose components determine the frequency of operation with great selectivity. The suggested values in this case are also commercial, which leads to an approximation of the frequency, but nothing prevents a more critical application from using adjustable components, always maintaining the relationships required by the double T.
