Money is an odd concept, really- it's a matter of shared belief in its value. As others have pointed out, in the age of fiat money, currency has no inherent worth. 1 US dollar represents, quite literally $1 worth of full faith and credit in the US government. Were people to, en masse, decide that this was worthless, and to refuse to accept US dollars, then all the dollars in your wallet- or bank account- would be literally worthless. Money has value specifically- and only- because people doing business agree that it has value, and will therefore accept money in exchange for goods or services, rather than demanding barter- that is, exchanging goods and services for other goods and services.
Bitcoins work in exactly the same way. They have value specifically- and only- because people agree that they have value, and will therefore accept Bitcoins in exchange for goods and services. Were people to, en masse, decide that Bitcoins were worthless, then they would be worthless.
Forbes shows his clueless with his comparison of money to measurements that "don't float." If he had a clue, he'd realize that, under the Bretton Woods system, national currencies *continually* float relative to each other in their value.