All communications rely on a basic principle: information is shared between two parties.
In the olden times communications was limited to talking (sound waves through the air), written messages that could be delivered, or even rudimentary visual signals.
The era of modern long-distance communications started with electrical signals over wire or RF that acted as a one-way broadcast mode, followed by two-way real-time long distance communications.
The problem with these methods, which persists today, is security. You don't know, for sure, if someone is listening in to your private messages. So encryption methods were created.
This is where entangled photons provide such a paradigm shift: through the herald.
When a pair of photons are entangled, they essentially share the same information. One photon can then be detected, or measured, locally (the signal photon), and the other photon sent across the long distances (the idler).
Through the magic of entanglement, when either photon is measured, both photons collapse instantaneously into complementary pairs. So the measurement of one informs the other party - the famous "spooky action at a distance." If some malicious party tries to intercept one of the photons, both parties know. Such is the basis of the ironclad security in quantum communications that deliver encryption keys or time - there is no way to circumvent this ironclad law of quantum physics (and if you figure out a way, then a Nobel Prize is waiting for you!)
But there is another subtle value of the idler photon - as a herald for the signal photon.
A herald brings to mind medieval times when a trumpeter in a bright outfit would signal the arrival of the king.
The same for the herald photon - it notifies both parties that an entangled photon is on the way. If it doesn't arrive (a lot of photons don't survive the journey) both parties know. But it also helps weed out the extra photons - from noise, or, malicious actors.
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