What does this wire do??

[Rookie2]

Do any of you all have a suggestion for a good book or reference to use for helping get a basic understanding of EWD's, nomenclature, measuring and understanding circuits. I had a basic circuits class 15 years ago, but I'd like to get a good reference to go through and learn/re-learn how to read/diagnose/test circuits and a get better understanding of what they're doing.

Thanks,
Rookie2