A sidechain is simply an alternate signal path that can be used to control an effect which is applied to the main signal path. It is typically with effects such as dynamics processing.

No Sidechain

When you use a dynamics effect which has no sidechain, then the control signal is derived from the main input, as shown in Figure 1.

Figure 1

Figure 1 - No sidechain

Internal Sidechain

The architecture for an effect with an internal sidechain is shown in Figure 2. Here you can see that it is possible to modify the main input before it is used to control the main effect. However what's really important to note here is that the sidechain processing doesn't effect the input to the main effect.

Figure 2

Figure 2 - Internal sidechain

As an example, let's consider a compressor with an internal sidechain with a high pass filter on it. This would allow you to set the filter to only let through high frequency plosives. In this case the presence of a sibilance will cause the compressor to reduce it's gain. However, the signal it's acting on has not been filtered - so the output has the full frequency spectrum that is seen in the main input, it's just reduced in gain whenever high frequency sounds cause the compressor to activate.

External Sidechain

An external sidechain allows you to control the main effect from a totally unrelated signal, as in Figure 3. Even though it's not shown here, it's possible that there could be further sidechain processing allowed on the external sidechain input.

Figure 3

Figure 3 - External sidechain

Let's take a look at an example where you want to use a ducker to stop a bass guitar line obscuring a kick drum sound. In this case, the bass guitar would be on the main input and the kick drum would be on the sidechain input of a ducker effect. Whenever the kick drum level exceeds the threshold level of the ducker, then the gain of the main effect is reduced and the result at output is the bass guitar with holes chopped out of it whenever the kick occurs. Note that you would want a ducker which only reduces the level of the bass, not mute it entirely - that would sound awful!