Sorry for the ignorance, but how should the json output be interpreted? Like, what does bugs: 0.28 mean in the example from the docs?

That's one of the Halstead metrics (https://en.wikipedia.org/wiki/Halstead_complexity_measures). It's all based on the number of operands and operators in your code. To make things simpler and practical, FTA combines these metrics with cyclomatic complexity (effectively the number of logical paths), normalizes them and produces a single number with accompanying 'assessment'.