This dataset is use to represent postcodes as polygons rather than points. It is unofficial as it was created for open source use by Mark's Blog (https://longair.net/blog/2021/08/23/open-data-gb-postcode-unit-boundaries/). While the Ordnance Survey (OS) publishes an official postcodes as polygon layer, Code-Point with Polygons (https://www.ordnancesurvey.co.uk/products/code-point-polygons), is not publicly available, and thus this approximation is used. See Mark's Blog for the process and limitations.

While these polygons do not exactly match Code-Point with Polygons, they are a way of showing postcodes as areas to more accurately represent reality than postcodes as points which OS does provide as public data in Code-Point Open (https://www.ordnancesurvey.co.uk/products/code-point-open). Note that some postcodes are discontinuous which is a feature of postcodes in the UK.