Every node maintains a smaller database of unconfirmed or pending transactions called the mempool (memory pool). It is the system used by cryptocurrency nodes to store data on unconfirmed transactions. It serves as a kind of holding area for transactions that haven’t been added to a block yet. A transaction is deleted from the mempool once it has been confirmed by being included in a block.
A broadcast transaction is one that is sent from a node to its peers, who then send it to their peers. This continues until the transaction has been broadly accepted and is prepared for inclusion in a block by miners. This buffer zone is essential since transactions are not immediately uploaded to the blockchain.
To make sure the transaction is valid, nodes will perform a number of checks, including making sure the signatures are accurate, the outputs do not exceed the inputs, and the money has not yet been used. It is rejected if it does not meet these requirements.
