What is a nano Sim tray?

what is a nano sim tray?

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Rudimentary answer

That's a tray for the nano SIM card. :-) Nano SIMs are even smaller than older generations and Apple started using them since iPhone 6, I believe.
I'm not sure whether there is even any phone at all out there any more that uses a larger SIM, except for dumb phones for seniors etc.
Lukas