Mempool also called memory pool also or transaction pool and its a temporary place where new bitcoin transactions go and wait until they are successfully added onto the bitcoin blockchain.
Bitcoin miner will choose and pick from this mempool which transactions to pull out and add on the new block of transactions which gets added onto the bitcoin transaction blockchain.
Approximately every 10 minutes a new block of transactions gets created by a bitcoin miner , therefore the number of unconfirmed transactions dropped, thats because this bitcoin miner was able to successfully pull out some transactions from the mempool and add them onto a new block. Normally miners will select those transactions which generate a high transaction fees for them.
Every node on the bitcoin blockchain keeps its own unique mempool of unconfirmed transactions, as different miners successfully mined new blocks they take transactions from the mempool of different unconfirmed transactions and successfully add them to a new block of transactions onto the bitcoin blockchain,
when that happens those transactions are no longer unconfirmed and instead become confirmed and that is what a mempool all about.
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