"Narrow complex" refers to the width of the QRS. One reference uses 120 milliseconds as the "threshold" between narrow and wide complex QRSs. Basically the electrical signal starts "above" the ventricles and as the signal is conducted through the ventricles, it is done so in a normal fashion, so the QRS is narrow. Examples would include sinus tach, a fib, and reentry SVTs (technically, sinus tach and A fib with RVR are SVTs). The treatments are differing for each situation. Here is a (fairly) concise looks at narrow complex rhythms: Narrow Complex Tachycardia
If the arrhythmia originates below the bundle of his (i.e., in the ventricles), the signal starts in the ventricles. Because of the "branches" nature of the conduction system in the ventricles, the QRS becomes wide, hence a "wide complex" tachycardia.
Aberrant conduction throws another variable into the situation, but we can leave that alone for right now.