A U-value describes heat transfer through a component under a temperature difference. In a like-for-like comparison, a lower value means less heat transfer. Establish whether the value relates to glazing, frame material or the complete assembled unit.
How this term relates to your project
For a garden door assembly, glazing-only information may look more favourable than information for the full frame and glass combination. Compare equivalent measures for the actual product configuration. Ask which size, panel arrangement and components the supporting information assumes. A product value also does not explain how the perimeter connects to the house.
Discuss that junction alongside insulation and airtightness. Treat the figure as one part of selection, together with operation, maintenance and shading. Have project requirements identified rather than applying a detached online number as a universal legal threshold for every replacement window or extension.