A bidirectional meter tracks the energy you export to the grid and the energy you import, and you're billed on the net.
When your panels produce more than your home is using, the surplus flows back to the grid. Net metering credits you for that exported energy against the energy you import at night or on cloudy days.
A bidirectional (net) meter measures both directions — export and import — and your bill is based on the net difference. In effect, the grid acts like a battery: you 'bank' daytime surplus and draw it back later.
Rules and export rates vary by country and utility, so check your local net-metering policy. Where available, it significantly improves solar savings because none of your surplus is wasted.
This is general guidance — figures vary by location, tariff, equipment and installer. Always confirm details for your own home before deciding.