Just culture is a principle anchored in Reg (EU) 376/2014: if a pilot honestly reports a safety event, the report itself does not become grounds for punishment. The safety investigation is kept separate from enforcement, so that people reveal problems instead of hiding them.
The logic is simple — aviation safety runs on data about what actually goes wrong, and that data only arrives if the person reporting is not afraid. The protection is not absolute, though: it falls away in cases of gross negligence or deliberate misconduct.