Extraneous means coming from the outside, like the extraneous noise you hear when you're in a theater and a train passes by. Extraneous can also mean not relevant or essential, like all the extraneous information in your long-winded science report.
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.