The mempool is where all valid transactions wait to be confirmed by the Bitcoin network. A high number of transactions in the mempool indicates a congested traffic which will result in longer average confirmation time and higher priority fees. The mempool count metric tells how many transactions are causing the congestion whereas the {mempoolsize} chart is a better metric to estimate how long the congestion will last.
In order to be confirmed, a transaction from the mempool needs to be included in a block. Unlike the maximum size of a block which is fixed, the maximum number of transactions which can be included in a block varies, because not all transactions have the same size.
Each Bitcoin node builds its own version of the mempool by connecting to the Bitcoin network. The mempool content is aggregated from a few instances of up to date Bitcoin nodes maintained by the Blockchain.com engineering team; this way, we gather as much information as possible to provide accurate mempool metrics.
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