I remember a lecture way back in second year at uni that mentioned this theory, but it is not something I have ever studied in great detail.
A quick google (we could end up down a rabbit hole here…) tells me it is the term for a phenomenon seem in a pair or group of particles that are generated, interact or share special proximity in such a way that the quantum state of each particle cannot be described independently of the others… Regardless of distance between them. Here is the link where I found this short explanation: https://everything.explained.today/Quantum_entanglement/
Hope this helps and if you have any other questions, ask away!
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