It is the ledger – a book of financial accounts – maintained by the nodes. When the nodes agree that you have signed the transaction to send one Bitcoin to your friend, they write that transaction into a “block” of larger transactions. That transaction simply involves instructions to debit your account one Bitcoin, and credit your friend one Bitcoin.
It is called a blockchain because each block of transactions is added to the chain of blocks before it, which creates a linear sequence of transactions. So, the nodes can see the transactions and the resulting credit/debit instructions that come out of each block. This keeps the ledger updated, clear, and protected from attacks such as “double-spend”, when a user tries to send one Bitcoin and, because of confusion amongst the nodes, can send the same Bitcoin again.
There are many different blockchains. Bitcoin was the first, but many have followed, making the process more efficient, cheaper, and scalable. Because of its reliability, eco-friendliness, speed, and cost, we have chosen to work firstly with NEAR. However, we will end up as a multichain protocol, meaning SWEAT can benefit from choosing which blockchain to use at any given moment.