As I would interpret it, human nature would refer to those inbuilt ways in which humans overwhelmingly naturally tend to act, even though it would be perfectly possible (physically, biologically) for them to act in other ways.
For example: almost anywhere you go in time or space, if you walk up to a guy who's eating dinner, yank away his food, splash his drink in his face, and walk away eating his dinner, he'll very likely get angry and, if he's big enough / you're small enough for him to have a chance, will start a fight with you. Or let's say, at the least, he'll get really angry. He wouldn't have to. He could just laugh and walk away hungry, looking for other food. He could hug you and thank you for teaching him a lesson.
But, overwhelmingly, he won't. He'll respond in that specific way, and that's human nature.
Culture obviously is super important, though. One can imagine that in a society that emphasized honor, respect, and perhaps fighting, he would fight you period, regardless of whether he had a chance. On the other hand, in an extremely deferential society, he might get very angry but be reticent to fight at all. Either of those would just be modifications of the innate reaction, though, and you still wouldn't see the bizarre alternatives listed above.