Light travels at about 186,000 miles per second. The distance light travels in ONE YEAR is a LIGHT YEAR. This is used because the distances to the stars are so far away you cant measure them in miles. It would be an impossibly large number. So "LIGHT YEARS" are used.
Basically space terminology--distances between planets---just a use of different terminology than regular years of 365 days in a calendar year. Calendars, incidentally, are different. The Hebrew people to this day go by an entirely different calendar and by the regular 365 day calendar. Chasia.