the product of number of turns and flux through the coil ........by maherbano

The phase shift is caused by inductance in the transformer. Any inductance from magnetic flux that fails to link both windings is called leakage flux, and the resulting inductance is called leakage inductance.

Synchronous Reactance (in a generator analysis domain) is and equivalent series per-phase inductance term (think per-phase winding resistance) and is mainly composed of the machine's per-phase leakage inductance (equivalent series inductance of primary and secondary flux leakage) and armature reaction (distortion in flux introduced by an armature current in a machine, once again on a per-phase basis; described as a series inductance). L_SyncReac=L_leakage+L_ArmatureReaction. That sort of touches the surface of synchronous reactance.

It stands for how does the primary and secondary winding magnetic fields connected firmly without much of leakage flux.

Inductance is inductance, and is not a function of frequency. Frequency affects reactance, and ultimately impedance, not inductance.