After you make and sign a blockchain action, what big step must happen before it can get put into a block?

After you make and sign a blockchain action, the big step that must happen before it can get put into a block is its broadcast and subsequent independent validation by the network's decentralized nodes. Once you sign a blockchain action, such as a cryptocurrency transfer, your wallet or node broadcasts this "transaction" to the peer-to-peer network. This means the signed transaction is sent out to directly connected nodes, which then relay it to their connected nodes, causing it to propagate throughout the entire network. As each network "node" – a computer running the blockchain software – receives the broadcasted transaction, it independently performs a series of "validation" checks. These checks include verifying that the transaction has a valid digital signature matching the sender's public key, that its format adheres to the network's rules, that the sender has sufficient funds to cover the transaction amount, and crucially, that it is not a "double-spend" attempt, meaning the same funds have not already been spent elsewhere. If a transaction passes all these validation checks, the node temporarily stores it in its "mempool," also known as a "transaction pool." The mempool is a waiting area for all valid, but unconfirmed, transactions that the node has received. Only after a transaction has been successfully broadcast to and validated by multiple nodes, and thus resides in their respective mempools, does it become eligible for a miner or validator to select it for inclusion in a new block.