We use our effect pedals consecutively, side by side in real world. But it doesn’t mean that they are in series, sometimes. And it doesn’t mean that we can’t use them in parallel fashion, in digital world.
In a traditional setup (pedals in series) your sound is processed consecutively by pedals. If there’s a distortion pedal at the very beginning, your sound will be distorted along the way. If there’s a reverb pedal at the end of the chain, the distorted sound will be reverberated.
In a parallel setup, you can split your signal into 2 paths wherever you want. That means, you can split your signal before it hits to a distortion pedal. First path goes through compressor, distortion and eq while the other path goes through chorus, pitch shifter, delay and reverb. These 2 separate signals then merge after the last pedal.
This may be hard to comprehend. And it may not be the most common use. But it opens up a whole new world.
Easier example would be like this. Guitar signal splits in two, goes through different amps and effects. The first path goes to left speaker while the second path to right.