The linebacker is usually the primary playmaker in a defense. While the defensive linemen occupy blockers, and the secondary covers receivers, the linebacker is expected to diagnose the nature of the play and either tackle the ballcarrier on running plays or, on passing plays, he either attempts to 'blitz' to pressure the quarterback, thereby acting as an extra defensive lineman, or he drops into coverage acting as an extra member of the secondary.
If the secondary (particularly the safeties) make to many plays then the linebackers are likely failing in their job and the offense is gaining to much yardage. If the defensive line is making alot, that is usually a good thing, but given that they are engaging offensive linemen directly, it is usually difficult for them to do so.
Exact responsbilities fluctuate from defense to defense as well as from play to play.