Poles and Zeros are concepts, used in computing analog and digital signal filters. They are also important in other areas of engineering. Poles and Zeros describe the transfer function of linear system well enough for many practical purposes, such as finding the needed values for resistor and capacitors in the filter electronics.
Signal filter (can also be some other devices) has input and output, and transfers input to the output through some function. As filter contains elements with "memory" (inductors and capacitors), the output depends on how the input value has previously been changing over time. The direct analysis of these dependencies would be very complex, but the mathematical method known as Laplace transform can transform this function into another function that is more easy to understand.
where is the transfer function. If multiple filters are connected sequentially, the computed output Y(s) becomes the input of the second filter, and only needs to be multiplied by its own transfer function to compute the output of the whole chain. This can be used to improve the quality of the filtering.
The transfer function is a function of the complex number, so it can be viewed as a 2D surface, one horizontal axis representing the real and another the imaginary part of the number. The  contains very nice plots of these surfaces.
This expression is infinite at the single point
This is the real part of the transfer function for this circuit. The imaginary part is zero. Hence the circuit has a single pole, located at the real (horizontal) axis. Sophisticated filters have more poles that often contain non zero imaginary part as well. However poles are normally symmetric over horizontal (real) axis. For instance, if the inductor would be connected sequentially to the resistor, such circuit would also be a filter, having two poles with non zero complex component, symmetrically placed on boths side of the real axis.
Places where the transfer function is equal to zero are similarly called zeros. See  for diagrams, how poles and zeros are placed in the most commonly used filters (Butterworth, Chebyshev, and Bessel). This reference also contains the actual wiring diagrams of the possible filters.
The applet (when active) allows to explore transfer functions with poles and zeros at various locations. The transfer function is defined in the graph at the top left where you can add and drag poles or zeros, the bottom part representing various characteristics of the obtained filter in the real time.
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