They are provided purely for human information. The same comment character is also used to append explanatory comments to most output lines. It is not sensible to attempt a complete description of this output format because every class of Object is potentially different and each can define how its own data should be represented. Objects may contain other Objects as data. This is indicated by an absent value, with the description of the data Object following on subsequent lines.
Indentation is used to clarify the overall structure.
Beyond these general principles, the best guide to what a particular line of output represents will generally be the comment which accompanies it together with a general knowledge of the class of Object being described.