"L'homme appartient au personnel."
"au" = "à + le"
"du" = "de + le"
So, they are prepositions to mean "to the" and "of the" respectively. Unfortunately, they're often required in French when there is no need to use them in English. OR, they are switched, so in an English sentence you would say "of the" but in French you need "au".
The worst part is there isn't really any rhyme or reason to it, no rules you can just remember. There are some references out there though about when some verbs require an "à" or "de" after them. Check out: