Extraneous means coming from or belonging to the outside—extraneous noise is what you hear when you're in a theater and a train passes by, extraneous wires bring your cable connection into the house.
In Latin, extra means outside, as in extraordinary "outside the ordinary," or extraterrestrial 'coming from outside earth.' (Bonus points––ding! ding!––if you knew that terra is Latin for "earth.") The meaning of extraneous also extends to more abstract things that come from the outside: extraneous details are ones that don't matter.
