Meaning of “with respect to rotor and stator”
From what I understood from your question you want to know what is "with respect" to refers to in a synchronous motor.

This means relative velocity. If two object are moving at same speed it appears like one object is stationary with respect to other. The phrase "excitation field in the synchronous generator is stationary with respect to the rotor" means that both field and rotor have same speed. 

For example let angular velocity of electric field is 40 rps. for synchronous motor, rotor is synchronous to electric field. Therefore speed of rotor is also 40 rps.

Here speed electric field with respect to stator is 40 rps. But speed of electric field with respect to rotor is 0 rps.