Most modern amps have an effects loop, but guitarists, especially those new to electric guitars, often don’t know how it works or what to do with it. When a guitar is plugged into an amp, the signal goes through a preamp, EQ, the power amp, and then to the speaker. If there is an effects loop in use, the signal is interrupted and sent to the effects pedal(s) via the effects send, and then brought back into the amp in the effects return. The signal then proceeds to the power amp and to the speaker.
Effects loops allow you to place effects between the preamp and the power amp. This works best with modulating effects such as chorus, delay, phase shift, or flanger. Modulating effects work well in an effects loop as they don’t pass through the EQ or preamp, and therefore aren’t colored by the circuitry. Many modulating pedals (phase shifters, for example) have inherent noise that can sound “swishy,” which the preamp accentuates. When your pedal’s operational noise is present in the processed signal, this would be a good time to test the stompbox in the effects loop and see if the noise is diminished.
You can place any pedal you wish in an effects loop, but be careful if you insert a distortion pedal in the loop. A distortion box is designed to overdrive the preamp stage, but if inserted into an effects loop, it could overdrive the power amp instead, and the results could be less than musical. Experiment with the placement of effects, both in a loop and straight in, and see which sound works the best for you.