Poles are when in the S domain the denominator goes to infinity. Were there is a zero the solution goes to zero. Now a pole will cause a change in the gain, the gain will start reducing by 20dB per decade, and this will show a negative 90° phase shift on the phase. Conversely, offset that pole with a zero, and so the zero has a positive shift in phase and a positive shift in gain over frequency. A very good example of a product or device that has a pole at the origin is a simple capacitor. As frequency increases, the impedance of the capacitor continues to reduce and reduce until, what is called the ESR zero, or the resistance, the inherent resistance. Then the designer will have the zero, so there is a pole at the origin and there is a zero at the ESR zero.