Purpose of Blocks in Ethereum

Blocks are containers of transactions. They are linked by a hash pointer to the previous block.

In general, it would be possible to approve every single transaction on its own. But this would make the consensus mechanism difficult. Hence, transactions are collected in blocks and validators vote for blocks instead of each single transaction.

A block is considered valid if it contains only valid (e.g., correct signature, no double spend, sufficient gas, etc.) transactions.

If one says, a transaction made it into a block, it means it became part of the history.