When looking at filters in the digital domain, a pole comes from the z-transform of the impulse response of a filter. The impulse response is the output when you feed a filter an impulse. A Butterworth filter is an all-pole filter that does not have a ripple in the passband. The passband refers to the range of frequencies that are able to pass through the filter.

Passbands

A passband is important for devices such as a radio receiver because they select for the frequency of the correct radio signal out of radio waves that are picked up by the antennae. The passband is the section that is passed by the Butterworth filter.

A passband transmission is an alternative to a baseband transmission. A baseband transmission uses line coding. This creates a pulse train or a PAM signal. This is typically used with non-filtered wires. However, if you'll be using filtered wires, you'll need a passband transmission. 

Passband transmissions are most often used in wireless communication. Passband filtered channels also use passband. A modem is the combined equipment for modulation and demodulation used by a passband transmission. 

Bode Plots

A bode plot is a type of log-log graph. The gain in decibels is plotted against the angular frequency logarithm with this type of plot. The bode plot is typically a combination of a bode phase plot and a bode magnitude plot.

With filters, one of the concerns is with not inducing peaking with the bode plot while also having the sharpest roll-off possible. A peaking filter produces a gain or a loss at a specific frequency. Peaking can be controlled when using Butterworth low pass filters in particular. Butterworth filters are useful in control systems because they minimize peaking. However, it's not necessary to eliminate all peaking with control systems because some peaking allows for less phase lag at lower frequencies.

Control Systems

In control systems, it's necessary to organize several poles and the Butterworth low pass filter is the sensible solution to this. There are other types of filters that are used in control systems, such as elliptical filters, but Butterworth are considered the best option in most situations.

A Butterworth filter is usually called a maximally flat filter because it does not have a ripple in the stopband or the passband. It achieves its flatness by not having a very relatively wide transition region from the stopband to the passband.