This page allows to change the orientation of a vessel in free flight. You can either specify the Euler angles of the vessel frame with respect to the global ecliptic frame of reference, or you can rotate the vessel around its pitch, yaw and bank axes.
The Euler angles α, β and γ define the rotation matrix R which transforms from the vessel local frame to the global ecliptic frame as follows:
More intuitively, you can rotate the vessel around its local axes by pressing the pitch, yaw and bank buttons. The pitch buttons rotate around the vessel x-axis, the yaw buttons around the y-axis, and the bank buttons around the z-axis.
If the vessel is part of a composite structure (e.g. a spacecraft docked to a space station), the whole structure will be rotated.
The rotation settings have no effect for vessels on the ground.