What do you mean by this ' static frictional force is self adjusting force'.Explain?

Static Friction arises from the fact that you basically join two objects together through their surfaces. When you, say, place a brick on top of a surface it..."enters" the surface!
Observing microscopically the two surfaces in contact we can see the joining of the two as a fitting of asperities of both: a "peak" enters into a "valley":

this phenomenon causes the two objects to lock together and oppose any kind of relative movement. Every time you push the brick these asperities or better, the locking between them will oppose the application of our force. So, macroscopically we can think that an "adaptive" or self adjusting force is at work opposing the applied one and changing continuously to match the applied force.

Static friction self adjusts itself to compensate the applied force and in doing so increases with it.
At a certain point (X in figure) the force applied to the brick will be so strong to break the interlocking (peaks and valleys) between the two surfaces causing the brick to start a movement.
Static friction loses its battle against the applied force!