What does grazing incidence means?

It means hitting the surface at such a shallow angle that it's

just about tangent to the surface.

When you're skipping flat stones on the surface of the pond,

you get the longest & best skips when the stone hits the surface

of the water at grazing incidence. Otherwise you get a plunk.

It means the incidence of a very small angle, typically with X-rays or neutrons.