"I downloaded a movie from the Internet and it says something about NL-Sub. What is NL-Sub and why should I care?
NL-Sub is shorthand for Netherlands Subtitles, which means you probably won't care about a movie having NL-Sub included unless you speak Dutch. Many of the movie downloads available through places like torrent sites include user-created subtitles so that people can watch the movie and get native language context for the movie's plot. There's a great deal of controversy about these subtitling efforts because they are often done without the movie studio's permission (as are the movie downloads themselves). Many other subtitle formats are available, but, possibly because the Netherlands are more lax on copyright violations than other places, NL-Sub seems to be among the most common designations in downloadable movies.