Just to add to tmpffisch's post of definitions:
Moderate turbulence - "there may be moderate changes in aircraft attitude and/or altitude, but the aircraft remains under positive control at all times..."
Severe turbulence - "abrupt changes in aircraft attitude and/or altitude - aircraft may be out of control for short periods - usually, large variations in airspeed..."
(AIP Gen 3.4 Pg 107 Appendix 2 Note 11)
You may not experience severe turbulence often, but any forecast of it , either in ARFOR or Sigmet, has me paying attention to the area in which it is forecast. Poor aircraft handling in severe turbulence, where accelerometer readings exceed 1.0g, can very quickly impose loads on the aircraft which approach its load factor limits.
Besides that, it's not much fun for you or the pax!