The IIR filter's transfer function is a ratio of terms.
The positions of poles (very large values) affects the stability of the filter:
The shape of the transfer function H(z) is determined by the positions of its poles and zeroes:
This can be visualised using the rubber sheet analogy:
Thanks are due to Jim Richardson for the rubber sheet analogy, which came to mind while he was an instructor officer at the Royal Naval Engineering College, Devonport.
Now the frequency response is the transfer function H(z) evaluated around the unit circle on the Argand diagram of z:
and since the shape of the transfer function can be determined from the positions of its poles and zeroes, so can the frequency response.
The frequency response can be determined by tracing around the unit circle on the Argand diagram of the z plane: