A blockchain is a ledger: a list of who owns what and who paid whom. What makes it unusual is who keeps it. Instead of one bank holding the book, many independent computers hold a copy and agree on every new page before it is added.
Each page is called a block. A block lists a batch of transactions and carries a fingerprint of the block before it. Change an old entry and its fingerprint no longer matches, so every later block exposes the edit. That chain of fingerprints is where the name comes from.
Accounts and keys
Nobody opens an account by filling in a form. An account is a pair of keys: a private key that only you hold and a public address derived from it. Signing a transaction with the private key proves the instruction came from the owner of the address, and anyone can check that proof without seeing the key.
This is why losing a private key matters so much, and why a wallet asks you to confirm every signature. The key is the account.
Why finance cares
A ledger that nobody owns can be read by everyone and written to by anyone who follows the rules. Payments settle on the same book the recipient reads, so there is no overnight reconciliation between two banks. Assets issued on it can move between companies that have never signed an agreement with each other.
Gleo runs on Robinhood Chain, an EVM network. The ideas above apply to it directly: addresses start with 0x, fees are paid in ETH, and every transfer can be looked up on the public explorer.