It's about some kind of object that apparently is the most important. The prefix hoofd- is added to indicate something is the main thing or most important thing:
- het hoofdonderwerp van de discussie = the main subject of the discussion
- de hoofdact van het festival = the main act of the festival
Keep in mind that voorwerp can only mean a physical object, not a goal (that's een doel (this is a het woord) both for a target and the thing used in sports).
Main object, meaning the centerpiece of a display of some sort? "The main object of the art gallery was his sculpture."
Or main object, meaning objective? "The main object of golf is to get the ball in the hole in the fewest strokes possible." "The objective of golf..." blah blah blah...
In Swedish we have the word "huvudsak", which usually means "that which is most important" (lit. "head thing"). It's often used abstractly, as in "Huvudsaken är att du mår bra" ("As long as you're feeling well...") Can hoofdvoorwerp be used like that? Het hoofdvoorwerp is dat je gezond is, or something like that?
That's hoofdzaak in Dutch, this literally means head case. It sounds like this word and huvudsak have the same origin. BTW hoofdzaak is the opposite of bijzaak (side issue). E.g. voetbal is de belangrijkste bijzaak in de wereld = football is the most important side issue in the world.