Kerbal Space Program

What is a prograde marker? In the "Go for Orbit" tutorial it states "Rotate your craft to align it with the prograde marker" in the "Orient Your Vessel" window. But "prograde marker" hasn't been mentioned in the tutorial yet, "velocity vector", yes, "prograde marker", no. I think 1.0.5 had small legend on the tutorial page when it was mentioned? But a picture of what a prograde marker is would be really helpful.

Also, it would be more accurate to guide the player to the "Maneuver Prograde" marker which is the blue one.

There is a legend in KSPedia under the navball section. But KSPedia hasn't been mentioned in the tutorials either, up to "Go for Orbit", not that I saw anyways. Seems to me KSPedia ought to be mentioned right after the KSC intro and before you are directed inside the VAB.