The "invisible hand" is a concept coined by economist Adam Smith. He posited that when people act in their own self interest that they also promote the good of the community as a whole. He only used the term three times but it was enough to take off and become part of the econ-lexography.
The theory states that if each consumer freely chooses what to buy and each producer is free to choose how to produce it, the market ultimately settles on a product distribution and prices that will ultimately be of benefit to the community.
The invisible hand also works as a balancing mechanism. Inhabitants of a poor country will work very cheaply. That's axiomatic. Entrepreneurs will build factories in poor countries with the motivation to make great profits. Entrepreneurs are also increasing the demand for labor the price will also increase. New producers become new consumers and local businesses will need to hire more people to provide for them the things they want to consume. Labor prices will eventually rise to the point where there is no advantage for the countries that are now doing business in formerly poor countries. Overall this helps the local economy.
It's important to note that Smith did not say that all self interested labor benefits society. He also did not assert that all goods are produced through self-interested labor. He was just pointing out that in a free market that people usually produce goods that are desired by their neighbors.
A reality of the free market is that there are countless opportunites to maximize one's own profit at the expense, as opposed to the benefit, of others. The tobacco industry is often cited as an example of this. Critics say that the social benefits (camraderie, "coolness", etc) are not balanced by the revenues received (which by any measure are ridiculously high).