The Term “effects loop” sounds complicated. And there are a lot of explanations out there that over complicate the effects loop. I’ll try and explain the effects loop as simply as I can.
When plugging straight into an amplifier the signal is essentially going in front of the amp and is being affected by the pre amp and power amp of the amplifier itself. We then us the EQ knobs on the amp to tweak the sound to our liking.
When using guitar pedals, we plug into the pedals first and then into the amp. This is called putting the pedals in front of your amp. Therefore we are affecting the sound of the signals before it hits the amp.
The effects loop is just like the loop that you’d put in front of your amp only the signal comes after the preamp and power amp.
Distortion pedals usually go in the front of the amp while delays and reverbs go in the effects loop after the dirt to maintain its qualities. Putting reverbs and delays in front of the amp, the sound tends to get messy.
This is the most simplified explanation I could think of. The concept goes more in-depth such as series vs parrallel effects loops etc. Read a more in-depth explanation of the effects loop here.
Here’sa visual explanation of the effects loop: