Imagine a shared notebook, where every transaction recorded in it is written simultaneously by thousands of different people around the world, not just by a single bank. No one can tear a page out of this notebook or change an old entry without everyone noticing. That, broadly speaking, is blockchain.
Each "page" in the notebook is called a block, and each block contains a group of transactions. When a block fills up, it's locked and linked to the block before it, creating a chain (hence the name: chain). To forge a transaction, someone would need to change not just the relevant block, but the entire chain across most participants simultaneously, which is nearly impossible computationally.
This is the key difference between crypto and money in a regular bank. In a bank, the bank itself is the entity that verifies and records every transaction. In blockchain, verification is decentralized among thousands of participants, so no single entity controls the information or can change it on its own.
This technology is the foundation for Bitcoin, Ethereum, and most digital currencies traded today, as well as many financial applications built on top of it.