A plant passport is an official label required for plants for planting, and certain other regulated goods, when they are moved between professional operators within the EU — and in some cases when they are sold to a final user. It travels attached to the plant, the batch or its packaging, and it says, in a standard form, that the goods meet the EU's plant-health requirements.
The legal foundation is Regulation (EU) 2016/2031, the EU plant health law, with the passport's exact format laid down by Commission Implementing Regulation (EU) 2017/2313. That is why a passport from a Dutch nursery and one from a Romanian grower look the same: the layout is harmonised across the Union.
It helps to be clear about what a passport is not. A phytosanitary certificate is a separate document, for moving plants across the EU's external border, into or out of the Union. A passport governs movement inside the single market between businesses; a phytosanitary certificate governs import and export. Confusing the two is one of the most common compliance mistakes, and one of the most avoidable.