dominant 7th chords
When a jazz beginner starts to improvise over a II-V-I progression, he generally plays arpeggios corresponding to the chords of the sequence.
In other words, he learns to play minor arpeggios over the minor chord (II), dominant 7th arpeggio over the 7th chord (V) and major arpeggio over the major 7 chord (I). This way the harmony is highlighted without taking risks.
In the long-run there is nothing exciting. That's why there is a nice trick used by number of jazz players to add smoothness to a dominant line. This is a little tip that makes all the difference which consists of playing a major 7th arpeggio over the V7 rooted on its b7 degree.