frozen-in property of vortex line
 

I am reading turbulence book written by Davidson. In chapter 2.3.3 the kelvin's theorem was explained. in page 49 in last paragraph this sentence was brought which is about the frozen-in property of vortex line:
"vortex line are frozen into a perfectly inviscid fluid in the sense that they move with the fluid."
unfortunately, I don't understand the meaning of this property and this sentence exactly.
does anyone know what the property is and explain it more?
thanks in advance.

well, "frozen" and "move" are actually somehow conflcting ...The right sentence should be read only as:

"vortex line into a perfectly inviscid fluid moves with the flow"

You can find explanations in any handbook of fluid dynamics