What are "Reliable Gold" and "Unreliable Gold" in Dota 2?

I noticed that when i hover my mouse over gold a pop-up comes up and in it there are: "Reliable Gold", "Unreliable Gold" and of course the amount of gold it takes to revive if you're dead at that moment.

What exactly is unreliable gold, and why is it in the game? I'd like as much info i can get about this.