Blockchain is a decentralized digital ledger that is used to record all transactions. Even though the concept is applicable to many industries, it became popular as the underlying technology of cryptocurrencies.
Every time someone buys, sells, or uses digital coins to purchase something, that transaction is recorded in the ledger. After that, the only way to alter the transaction is to overwrite all subsequent transactions, which is nearly impossible since copies of the whole ledger, with millions and millions of records, are stored on computers all over the world.
While banks operate only during certain hours of the day, which may take cross-border payment days to be processed, blockchain technology allows transactions to be validated within minutes, possibly even seconds. By removing the middleman, it should significantly decrease transaction fees.