Input/Output Impedance of DC/DC converters and Impedance matching
What I would like to make clear is what exactly do we mean by
  input/output impedance, what the above Z_out,Z_in are supposed to
  represent



\$Z_o\$ is the external impedance connected to the buck converter's output.
\$Z_i\$ is the impedance presented by the buck converts input to the power source.



  how can I achieve impedance matching with the source, when based on
  the above equation, by changing the duty cycle I can merely change the
  ratio of the converter's impedances


The impedance seen by the source is the impedance presented by the buck converter, \$Z_i\$ and, in turn, that presented impedance is dependent on \$Z_o\$ and the duty cycle used by the buck converter.

In other words, the input impedance presented to the power source by the buck converter loaded with \$Z_o\$ is: - 

$$Z_i =\dfrac{ηZ_0}{D^2}$$