I am a newbie in CFD. Since I begin my study in CFD, I have faced the term "Wall Shear Stress" on numerous occassions on several articles. And the phenomenon seems to me to be playing a very big role in Fluid Dynamics. But reading several textbooks and related articles, I could not grasp the real significance of the phenomenon. Would someone please explain me the significance of "Wall Shear Stress". Thanks a lot for your attention.

The wall shear stress is simply the viscous force acting on a wall.

to be more precise: the wall shear stress is the force per unit area, acting in tangential direction on a wall surface.

It's not surprising that you have encountered this term a lot. Much of fluid dynamics is focused on the effect of flow on structures (e.g. lift and drag on a wing).