What are kilometers and explain in words.

A kilometer is a unit of length or distance.  It's defined as
the length of 1,000 meters.

To perhaps give you a better feel for it . . .

         1 kilometer = about 1,094 yards

         1 kilometer = about  0.621 mile

         1 mile  =  about  1.61 kilometers

Almost every place in the world outside of the USA posts their
distances and speed limits on their road signs in kilometers
instead of miles.