'Human nature' refers to something that humans -- all humans -- will naturally do, or skills and wants that they will naturally have.
Many people try to claim that various behaviors are in fact 'human nature'. Unfortunately, there is very little in the way of a scientific basis to most of these claims. But by looking at the field of anthropology we can look at things that probably are human nature (as opposed to American nature, or Western nature). Anything that appears in each and every human culture could be considered human nature. Things like love, anger, religion, making art, fighting, making tools, breaking things with them, and overeating are all part of human nature. Probably.
We can debate most of these -- romantic love isn't valued in some cultures, religion is falling out of fashion in many parts of the world, some would like to think that we can rise above anger without loosing our 'human nature', we may be reluctant to think of someone too mentally disabled to make tools as 'sub-human'... And so it goes. Most of us are happy to let 'human nature' mean 'human average'.
Generally, we think of human nature as being genetically determined, although many people dislike basing too much of our humanity on genetics. Speech, for example, is clearly part of human nature, but there is a long standing debate as to how much of it is pre-programmed.