Generally speaking, the currency is issued and supported by a certain government. In other words, the value of a country's currency is ultimately tied to the credit of that specific country.
However,
the biggest characteristic of Bitcoin, and the fundamental nature of bitcoin, is decentralization. In theory, it is a rare and secure thing that guaranteed by algorithms in the virtual world, which not bound to anything.

The unique about bitcoin, in the words of its anarchist supporters, is that even the dollar could lose its credibility and its value as the United States declines, Bitcoin will never be affected like that.
The credit support of Bitcoin is the power of faith, which means that the more people use it, the more valuable it is, and the more secure it will have. Thus, the value of Bitcoin will increase after the number of users getting expend, the security will go to a higher level as well.
This will be going into a continuable circle. The core feature of blockchain technology is that it can transfer people's trust from the authoritative center to trust in themselves as a whole. In other words, bitcoin's credit is backed by the people who use it, as well as its source of value.
Therefore, personally speaking,
the core problem which Bitcoin trying to solve is to create a trusted digital certificate that can be used as a currency.
The technical basis of Bitcoin is cryptography which guarantees its credibility. The cryptocurrency will not exist as a currency once the encryption is broken, this is why we call the digital certificate as "cryptocurrency".
Bitcoin has three characteristics, which based on these three things, it can be trusted and treated as money.
First, Bitcoin cannot be forgedEvery bitcoin can be traced back to its source, and all of the bitcoins come from the rewards that the miners get. The miners have to build blocks to get the rewards which uneasy and require a certain extent technology to do, thus, you can't forge bitcoins.
Second, Bitcoin is hard to be stolenBitcoin can't be easily stolen, or conversely, it makes it impossible for you to steal someone else's bitcoin, you have to spend your own money. Because to get other people's money out, you have to own another person's private key and it is very hard to get.
Third, bitcoin will not be issued in massive numbersThe rate of bitcoin issuance is very stable, furthermore, the number gets halving every four years and eventually ceasing to grow. This indicates Bitcoin will no go to be inflation caused by government abuse like fiat do.
It's important to note that bitcoin can't have an entity, there's no way to pull a Bitcoin out of your pocket, the entire transactions have to be done over the Internet.
How can there exist invisible money?Well, money is supposed to be intangible. The money of those entities is a waste of materials. I the past, the money have to be made into entities due to the lack of advanced technology.
When we were kids, we had to buy things in cash, otherwise, we couldn't prove that we had the purchasing power, and it was only through physical money that we could make sure the other person received the money. However, after the development of the banking industry, pay by credit card gradually replaced some people's buying habits.
Now, physical credit cards became unnecessary after the development of the internet. The central billing system now is in place which makes the transaction goes secure and stable.