A light-year is a way of measuring astronomical distances in space. That doesn't make much sense because "light-year" contains the word "year," which is normally a unit of time. Even so, light-years measure distance.
Here on Earth we are used to measuring all distances in either inches/feet/miles or centimeters/meters/kilometers, depending on where you live and what method of measurement is used. Thanks to school, most people know how long a foot or a meter is — we are comfortable with these units because we use them every day. Light-years, not so much. Same thing with miles and kilometers — these are nice, human increments of distance that our minds can easily understand. Light-years, not so much.