Whenever you send someone Bitcoins, the transaction goes through different computers running the Bitcoin protocol around the world that make sure the transaction is valid. Once the transaction is verified it then “waits” inside the Mempool (i.e. in some sort of a “limbo” state).
It’s basically waiting to be picked up by a Bitcoin miner and entered into a block of transaction on the Blockchain. Until it is picked up it’s considered an “unconfirmed transaction” or a “pending transaction”. A new block of transactions in added to the Blockchain every 10 minutes on average.
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