in the function of henle's loop. its written that ascending loop is impermeable to water but how come it dilutes the filterate???

The Ascending loop of Henle is impermeable to water but permeable to electrolytes so the electrolytes move outside the loop & the amount of water remains constant resulting in the low concentration of solute inside the loop.  Thus dilution of filterate takes place. Consider the figure given below-

In the above figure, NaCl (electrolyte) is moving out of the ascending loop resulting in the dilution of filterate.