Basically, it means that some people earn a whole lot more than other people.
It's also called 'income disparity' (which simply means differences in income)
For example: almost everywhere in the world, women earn less than men, even when they're doing the same kind of work. (That can be called 'the gender gap' in income.) In many places, race has a similar effect.
Another aspect of the income gap/income disparaity can be the overall distribution of income, where perhaps 10% of the population earn 90% of the income - which is obviously no fun if you're in that other 90% of the population.
There is evidence that the income gap has been growing in recent years in the USA
(see for example www.nytimes.com/2007/03/29/business/29tax.html )
The general Wikipedia article has some more points as well: en.wikipedia.org/wiki/Income_disparity