Quantum entanglement happens when two or more particles are created or interact in a way that ties their properties together. Once entangled, measuring a property of one particle, like its spin, immediately fixes the corresponding property of its partner, even if that partner is on the other side of the galaxy.
This is not a signal traveling between them. Nothing is sent, and no information can be transmitted faster than light this way. What is strange is that before the measurement, neither particle has a definite value at all. The pair is described by a single shared wave function, and measuring one collapses the whole system at once.
Einstein famously called this “spooky action at a distance” because it seemed to break the rule that nothing travels faster than light. Decades of experiments, including ones that closed every loophole skeptics raised, have confirmed entanglement is real. It now underpins quantum computing and quantum cryptography.