Since the prize will be awarded in Bitcoin its probably a good idea to understand what it is.
Bitcoin is a digital currency. It has no physical form, such as bills or coins. Nor is it controlled by a government or bank like traditional currencies. Bitcoin is controlled by a peer-to-peer network vs. a central bank and is secured through significant cryptography. BTC is shorthand for currency, similar to the way USD is shorthand for the US Dollar.
It was released to the public in 2009 by an individual/group named Satoshi Nakamoto. More on that later.
To buy/sell Bitcoin you first need to a Bitcoin Wallet. This is where your "keys" are stored for the Bitcoins you own. You can obtain a Bitcoin Wallet from an online service, software (desktop or mobile), or hardware such as a USB stick.
Once you have a Bitcoin Wallet you can use a Bitcoin Exchange to buy or sell Bitcoins. Many online merchants now accept Bitcoin for payment. There is a small fee for each buy or sell transaction.
Other digital currencies such as Ethereum, Litecoin, Ripple. Bitcoin was the original and is still the most popular.
For the purpose of the treasure hunt, the private key from a Bitcoin Wallets is important. That is the way you will access the prize. Once someone gets 400 keys, they can combine them to create the private key that you can claim the prize with.