What does “legally binding guarantees” mean?

The Decision of the Heads of State and Government at the European Council on our guarantees:-

• Will be legally binding.
• They will constitute an international agreement, lodged with the United Nations
• The agreement will take effect on the date of entry into force of the Lisbon Treaty.

This process was followed in December 1992 to address Danish concerns following their first referendum on the Maastricht Treaty.

The Edinburgh Decision was an international agreement - binding in international law, as are the Irish guarantees.