One output of this document when another document uses it as a block: what arrives at Value, of any kind, which that document reads by this block's name (as block.Name). Main makes it the document's main output: what the editor shows first, what a run without the editor measures, and what a parent reads when it names no output. With one Output, that one is the main output; at most one is marked Main. Its name is unique among the document's Outputs, without a dot.
What this output is: any block's result, of any kind.
Whether this is the document's main output.