What does ‘bypass the Ethereum mempool’ mean?

From the Harmony hack:
https://etherscan.io/tx/0x2f259dec682ccd6517c09b771d6edb439f1925e87b562a72649a708fdd0511e1
The info on top 'Private transaction' says:
This transaction bypassed the Ethereum mempool and was sent directly to miners.
What does this mean?
submitted by /u/fap_fap_fap_fapper
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