What is the purpose of the piece of filter paper inside the 400 ml beaker?

The purpose of a filter paper inside a beaker is to retain the suspended solid particles found in the solution.

The retention capacity of the filter paper will depend on the size of the pores of the filter. Thus, for example, if the pores of the filter have a diameter of 1 mm, particles with a diameter greater than 1 mm will be retained, and the rest of the solution will pass through the pores.

The removal of impurities through filters is a physical separation process.

particle   p is an electron and particle q is a neutron