Think of it this way. Rich people are hoarding all the wealth, right? Well if you take the average, or mean, of all incomes, rich and poor, you get like $75,000. All that number tells you is, add up everyone's incomes, divide by the # of people, and if everybody made the exact same amount, that's how much they would make. But if you say I want to line all the people up in order of richest to poorest, and stop in the very middle, the very middle guy of all the people lined up, what does he make? Well, median income is only about $50,000, a hell of a lot lower than that $75,000 average, so ya know ownership of wealth is top heavy, there's millionaires and billionaires bumping the average up while almost everyone else is making far below that $75,000 figure.
Another way to think of it is, let's say one guy made $1000 and 99 guys made $1 each. Find the mean, (1099/100=) it'll show an average of about $11 per person. However, line those people up richest to poorest, $1000 guy is over there chilling on the end, and then a whole line of $1 guys, the median is $1 because that's what the middle guy is making, but you wouldn't have known that had you just gone with the rosier picture the average painted and assumed most people were making somewhere close to $11.