In simple terms, 'holes' here mean lack of electrons to fill fully up the bonds and considering a particular place within such a semiconductor, the same situation exists everywhere in the neighbourhood of this place, so it takes time for an electron to break out from a tied position and fill up the next place also lacking an electron; (so it turns out the hole conduction also involves the moving of electrons just like in the case of the electron conduction but you surely know already this).
Some links: **broken link removed**
and some more details on other conditions like temperature etc: **broken link removed**
rgds, unkarc