A blockchain is a decentralised, distributed digital ledger consisting of records called blocks that are used to record transactions across many computers. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data (generally represented as a Tree, where data nodes are represented by leafs).
Researcher Bettina Warburg explains blockchain technology to 5 different people; a child, a teen, a college student, a grad student, and an expert.
The timestamp proves that the transaction data existed when the block was published to get into its hash. As blocks each contain information about the block previous to it, they form a chain, with each additional block reinforcing the ones before it.
Therefore, blockchains are resistant to modification of their data because once recorded, the data in any given block cannot be altered retroactively without altering all subsequent blocks.