entanglement
A link between qubits so that their results stay connected, however they are read.
When qubits are entangled, they can no longer be described one by one. Reading one tells you something about the others. Quantum programs use entanglement to coordinate many qubits at once. It is a large part of what lets quantum computers do things ordinary computers cannot. It is also fragile and easily broken by disturbances.
Think of it as
Like a pair of gloves posted in two boxes: open one, find a left glove, and you know the other box holds the right one.
Example
A quantum program entangles 20 qubits so they act as one system. If stray heat disturbs just one qubit, the link weakens across the group, and the final answer becomes less reliable.