Incidence is the angle of the centerline of the main wing and/or horizontal stabilizer to the thrust line. It can be used to control ballooning at higher speeds, or other unwanted flight characteristics. Look at a profile pic of a Fokker Dr.1, D8, or D7. See how the leading edge of the H. stab is raised relative to the thrust line? That's positive incidence. In this case, it forces the leading edge of the wing down in level flight, keeping the tail flying when the wing stalls, and allowing a more gentle, slower stall and an easier recovery.

Washout is when the wing tip leading edge is lowered in relation to the root L.E. (twist). This allows the wing to stall at the root first, which is much better than the tip stalling first. Washin is the opposite, and I don't know any plane that would benefit from it.

Dr.1