What Does This Red Line Truly Indicate?

[Sovran Nila]

What does this red line truly represent? What is "occurring" when the red line is introduced as a visual cue?

 

[Papasmurf]

That red line was present in the manga too and was accompanied by a "crrk" effect indicating Gohan's last line of restraint had fallen. It's showing his rage finally snapped and broke past his limits.

 

[Hector]

Communist subliminal message. Cell is the symbol of capitalism that must be defeated, which is why he has the looks and manners of a businessman.