This is the 671st most frequent Polish word.
domu
"Domu" means "home" or "house" in the genitive or locative case in Polish.
The word 'domu' is used here to indicate location, expressing that the person is presently at home; it is in the locative case.
Jestem w domu.
I am at home.
In this sentence, 'domu' is again in the locative case to express the location where there is no one.
Nie ma dziś nikogo w domu.
There is no one at home today.
Here, 'domu' is in the genitive case due to the preposition 'do,' indicating motion towards a place, in this case, home.