“Index is Out Of Range Error”

What does this error mean – specifically the part which says this index is out of range. I have around 500 of these errors in my script and I need to clean them up.

(Turn 2 cccc (copy)) There’s a problem with ‘Action Block 1’ in reaction ‘Loop 3 Colours’ (from ‘script is initialised’ listener) >>

>> Index out of range