Can anyone please explain the meaning of this para( frm. Mechanics of Flight, pg.no.34)?

on the windward side of a large building, or a hill, the wind is deflected upwards, and on the leeward side it is apt to leave the contour altogether forming large eddies which may result in a flow of air near the ground back towards the building or up the far side of the hill, that is to say in the opp direction to that of the main wind. even when the surface of the ground is comparitively flat, as of the avg. airfield, the wind is retarded near the ground by the roughness of the surface, and successive layers are held back by the layers below them- due to viscousity-and so the wind velocity gradually increases form the ground upwards. this pheno. is wind gradient. when the wind velocity is high it is very appreceable ,and since most of the effect takes place within a few metres of the ground it has to be reckoned with when landing.