A combo is entered as one instruction with a net debit or credit rather than separate orders. The exchange fills all legs or none, which removes the possibility of ending up with half a position.
Stock-plus-option combos matter for buy-write and delta-hedging: entering stock and option together locks the effective basis instead of leaving the hedge exposed while you chase the second fill.
Example: a buy-write on XYZ at $50 selling the $52.50 call at $1.10. Entered as a combo at a net debit of $48.90, you either get stock at an effective $48.90 or nothing. Entered separately, the stock might fill at $50.15 while the call decays to $1.00, turning a planned $48.90 basis into $49.15.
Related: complex-order-book, spread-order, legging