The pixel unit is maybe a bit misleading here because in the context of Paper.js canvas, it is a factor of the view zoom and the scale of the elements...
So think of it as a value that you can compare to other Paper.js items, a rectangle for example.
Here is a sketch
demonstrating how this value can be compared to other shapes.
So if you set up a text with a font size of 25, its body will have the same height as a rectangle that has a height of 25.
Note that in this sketch, for convenience, I used the same value for `fontSize` and `leading`.
The leading can be compared to the `line-height` in CSS.
As mentioned in my answer to your other question, this is all related to how the web Canvas API works.
If you want to go more in details, maybe the best is to dig into the source code and check out the Canvas API documentation.
Hope it helps.