What does"And mankind is naught but a single nation" mean exactly?

It means that the division into many countries was due to our immature age in the past . Why there are such huge among tiny, rich among poor, advanced among backward countries? And this disparity is more threathening with various nations embark on arm race.
Now due to the coming of age, mature human beings realize that, all people must join hands to save ourselves from global calamities and building equality and justice on the whole world.
Usually, high ideals would be realized through a revolution or through a proper education. And with our maturity, we also realize that gradual advancement through education is better than a revolution.
The ways of implementation may differ from vatious groups of people, but the ideal of world unity is clearly a must for our Age. And we can rationally hope that the mature human beings know how to consult for reaching the best plan and result.

probably people are the same where ever you go.

There came a day when it was easier to separate one another through ethnicity, culture, beliefs, and so on... rather than realize that we're all the same species.

All borders and barriers are essentially artificial. Most people, whether or not they take a literal view of Genesis, recognize that we all sprung from the loins of the same parents and should treat each other as our kiy=th and kin: which we are.

all that humans are is one nation. like we are just one species out of thousands.
although that sound a little depressing but i guess it depends how you look at it.

That the concepts of patriotism and racism are a little more tightly linked than we might care to imagine.

That we are all Marxists at the core.

It means that our world is one nation and the divisions in it are not real in a way and that if these nations were truely seperated they would all cease to exist.

I would disagree on the 'naught' - just as rabbits are a nation or species unto themselves - all animals are thus, all humans are part of the same species, but we are more than a species - we, as all life are, individuals. :)