When scientists talk about the "flatness" of the universe, they are referring to its overall geometry or shape on a large scale. A flat universe, in this context, means that the geometry of space-time is consistent with what is predicted by certain cosmological models.
The 0.4% error mentioned indicates the level of uncertainty associated with the measurement of the flatness of the universe. It means that the measured value of the universe's flatness could deviate from perfect flatness by up to 0.4%. This error represents the range within which the true value is likely to fall.
However, it's important to note that this error does not imply a probability that the universe isn't actually flat. The statement doesn't suggest a 0.4% chance of the universe being non-flat. Instead, it conveys the precision and confidence level associated with the measurement.
In scientific research, errors and uncertainties are inherent due to limitations in measurement techniques, observational data, and theoretical assumptions. The reported error value allows scientists to quantify the level of confidence they have in their measurements and calculations.