pointwise constraints

I am wondering if someone can explain what pointwise constraints are.
When should I use them?

The most important thing I want to know is, what does it mean to use pointwise constraints with an equation of x*fsi.nX+y*fsi.nY+z*fsi.nZ? I understand that when I use a pointwise constraint of x*fsi.nX+y*fsi.nY+z*fsi.nZ, it means x*fsi.nX+y*fsi.nY+z*fsi.nZ=0. But what does this physically mean?

In other words, if I define a prescribed mesh displacement, say x=0, then I know it means that the mesh does not move in the x-direction. Similarly what does the pointwise constraint signify/represent?

I am hoping someone will answer my question.

Thanks,
Sirisha