Quantum error correction

Qubits get their values knocked off by tiny disturbances such as heat or electromagnetic noise. So the method is to tie many real qubits (physical qubits) into one bundle and use it as a single reliable qubit (a logical qubit).

If some in the bundle are wrong, they can be fixed by comparing with the rest. The problem is that one logical qubit takes a very large number of physical qubits. Cutting that number is now the most intense race in the field.

It is the same idea as saying something important three times, and the listener trusting whichever version came up most often.

It is a hurdle you must clear to run large, useful calculations. An announcement of "we built this many logical qubits" is often bigger news than the number of qubits.