Playing notes or sequences of notes that aren't obviously in the chord being played, or even in any of the corresponding musical scales.
For example, in the twelve bar blues progression played in the key of C, when you get to a G7 chord you might play an E-flat.
The notes of the G7 chord are normally G, B, D, F, and A would fit nicely as well but then it'd be G9. And probably the most common scale you would play over this chord is a G mixolydian scale (G scale with a flat 7). None of the chord or scale notes is E-flat.
But the E-flat can be seen as making sense if you play a B chord over the G chord's second note. The E-flat is then the 3 of the B chord. If you try the B over G on your instrument, omit the D and F notes which will create more dissonance than would be aesthetically pleasing to most people.