The term slacker, in contemporary western culture, applies generally to high school, college, and young people who underachieve at school or work. To other people, it appears that slackers don't do very much actual work, schoolwork, or much of anything. The term is usually associated with grungy MTV-watching Generation X types with their dismissive sayings such as "Whatever" and "Never mind".
To slackers, educational institutions such as high schools and colleges do not adequately prepare young people for the Real World, so why bother making an effort to get good grades?
Slacker is the name of a 1991 film written and directed by Richard Linklater, which popularized this word in the 1990s in its Gen-X sense.