if the richest 85 people give away all their money? I once heard a fact that the richest 85 people are as wealthy as half the world's population put together. My question is, what does that mean?

Does it mean that there is some feasible method in which these 85 people can double the wealth of half the world's population? What would happen if they decided to do so? How would they go about doing this?
It seems safe to say that the bulk of that wealth consists of equity (stock) holdings, not "money." (For example, Bill Gates owns a lot of share of Microsoft.) 

Let's assume that the top 85 people wanted to redistribute their wealth voluntarily.


If they tried selling that equity all at once, the price of the equities would likely collapse (who would pay a lot of money for those shares, if everyone know that they were selling all at once?) The amount of money to redistribute would be a lot less than the current estimate of their wealth (which uses the current stock prices). The logistics involved with the transfer would probably consume most of of the transferred wealth. (If it were easy to transfer wealth to individuals in poorer countries, aid programmes would have been more effective than seen in practice.) That is, some people would be wealthier, but it would likely be intermediaries skimming off the cash transfers. 
Logistically, there would be no way to transfer all of those shares to half the world's population. (They would need something like a brokerage account.) And even if that were somehow accomplished, the recipients would likely dump their shares, and their value would collapse. Brokers would probably end up getting all of the value from their transaction fees.


In other words, there would presumably be a transfer of wealth, but it would be a lot less than what is implied by current market prices.

This of course raises the obvious question of why they would do this voluntarily; the top 85 richest people did not get that way by dumping their wealth on random strangers.