Think of a blockchain as a publicly shared Excel sheet.
Anyone can access, read, save, and share this Excel sheet. No one can delete or edit its content. You can only add a new row(s) to it, which everyone can then read and access afterwards.
Those newly added rows usually record transactions. Those transactions are saved in a unique, encrypted way, and called blocks.
Those blocks are linked to each other on what is called a chain. The final ledger is called a Blockchain. It rose to fame with Bitcoin, being its most ‘famous’ application at the time
You don’t need a central trust authority to tell you those blocks are legit, not fake. The encryption used in building the blockchain itself takes care of that.