Axis
An axis is the imaginary line around a craft can rotate. Each object has three axes around a craft can rotate. The command pods apply the force to rotate with SAS, RCS and TVC assisting to rotate. While SAS support rotating around all axes every time, RCS only if there are thrusters at the side of the axis and TVC can only assists rotating orthogonally to the engines thrust.
The size and mass of the craft affect the ability to rotate. In general, the further away of the axis a mass is the more force is needed to rotate it around the axis. So rolling is in most cases not a very big problem for a rocket while yawing and pitching without available TVC or RCS is relatively hard.
By default the viewpoint of rotating is from the root command pod so the described movements here are only the default cases. It is possible to change the viewpoint from the rotation, by changing the part from which it is controlled. To do so right-click a part and select “Control from here”. Only command pods and docking ports are supporting this option and it will rotate always around the center of mass (SAS) or center of thrust (RCS, TVC) not around the selected part.
Yaw
A motion around the yaw axis results in moving the nose to the left or right and the tail to the other side. The axis is orthogonally to the display's plane by default. Wide or long crafts make rotation harder while the depth isn't very important.
Pitch
Pitching will move the nose downwards and the tail upwards and the axis goes from one side to the other. Long or depth crafts will damp the rotation. A very wide craft won't impact the ability very much.
Roll
Rolling allows the craft to rotate around its central axis. A very tall craft or aircraft won't change much, but a wide or depth craft will impact it. In most configurations rolling couldn't be supported with TVC. Placing an engine on the rockets side however allows to roll with TVC, but it is not very common. Because of this a rocket can easily start rolling if there is not enough SAS available to counteract and boosters on the side aren't placed at exactly the same height.
With rolling and pitching aircrafts can do more effective turns to the left or right than only with yawing. To do so roll to the desired side and then pitch upwards.
