Hello and thank you for the question. In hindsight, this could have been worded a little differently. In many old school dungeon games, you roll 1d6 to see if there is a monster in the room. Affirmative on a 1 (sometimes 1 or 2). Other systems: affirmative on a 5 and/or 6.
"If negative," means, if there isn't a monster in the room (per your chosen game's particular rules), then roll 1d6 to see if there is an Event or Feature.