I am not at all adept at explaining the technical aspects so anyone can feel free to correct me, but in a nutshell...
It is an electronic currency that is decentralized - not like the tangible fiat money/national currencies like US Dollar or Euros or Yen, etc. There is no physical paper/tender like a minted coin or paper currency, so there is no central authority in control of it like a government bank. It is all traded or exchanged electronically.
You can buy it with regular currency and you can sell it for regular currency (or you can trade between thousands of different cryptocurrencies) but all of that is done online in exchanges - similar in concept to a stock exchange or to a foreign money exchange. Their values can be pretty volatile and you can make a crap ton of money or lose your shirt depending on how much you buy or sell it for. Just over a year ago at its peak one Bitcoin was worth close to $20,000. Today it is worth about $3,500. A few years ago it was worth less than $100. So it goes up and down! It is all based on supply and demand and what people are trading it for. But that's just Bitcoin. Some other cryptocurrencies are worth crap and losing value; some are scams; some are not and are increasing in value. But as more and more people get fed up with decreasing values of their national currencies and turn to this as an alternative way to buy and sell goods - it has a chance to someday spike again in value.
You can 'own' or spend and trade fractions of a Bitcoin divisible down to 0.00000001 (which they call Satoshis - named after the mystery guy who 'invented' Bitcoin) - so even though a whole Bitcoin is worth thousands of dollars - you can use it in values more closely in line with what you are used to in terms of dollars and cents.
I've invested a small sum into it and I also earn small bits on web sites that pay you to click on advertising or take online surveys, etc. Similar in concept to Fusion Cash... I figure that my $20 today could either drop down to next to nothing, or it could someday become $20,000+. There are some early adopters who have become millionaires simply because they got in at the right time and saw the potential.
It is all tracked and documented online in what they call the blockchain - so the entire 'ledger' of each Bitcoin (or whatever cryptocurrency you are dealing with) in all of history is tracked and verified from multiple independent [decentralized] sources through a series of complex computer calculations and equations. As multiple computers confirm transactions through that process, it is written to the blockchain and essentially becomes "written in stone". Each time a certain number of transactions are completed and independently verified, that becomes a new "block" added to the "chain." If someone were to try to go back and hack into a prior verified transaction - that variance in the blockchain would be detected and flagged as inconsistent with what has already been established and verified through multiple independent verifications. (((This is the part that is too technical for my limited ability to explain, but hopefully I am both accurate and articulate in trying to explain it in my own simplified understanding)))
You will be hearing more and more about the Blockchain concept as I think it is going to become more pervasive in society in coming years and decades. Right now it is mostly used to track cryptocurrency transactions, but there is talk (and many aspects are already in motion) of using it to store/verify/keep track of so many other aspects.
I won't go into those specifics (again, not sure that I would be able to), but many believe that the blockchain concept will be more revolutionary than the Internet itself.
I only got in about 18 months ago and still have a lot to learn. If you are more interested, there is a podcast called the Bad Crypto Podcast that has a lot of good information on the subject. If you go back to their very first few episodes, they explain a lot of things quite well. I have some other general information and informational links on my web site [not allowed to promote it here as it also has non-FC referral links] but there is a lot out there.