Your modular synth may not contain every sound processing modules you may want to use (yes – shocking, isn’t it?). Therefore, sometimes you might want to send a signal outside your modular system, process it through an external effects device, and bring it back into your modular for more processing. This going out/coming back in is referred to as an effect loop. You’ll find the same idea in mixing consoles, guitar amps, and the such.
The trick with modular synths is that their internal signal levels tend to be much higher than those used by external effect equipment, so a modular effect loop will usually have level matching circuitry as well.