Out of doors means outside, in the open air rather than inside a building. When you play out of doors, you're running around in your yard or a park instead of staying inside your house. A family might eat dinner out of doors on their patio on a warm summer evening, or a class might have a lesson out of doors under a big tree.

The phrase sounds a bit old-fashioned because it was more common in earlier times, before people used the modern word outdoors (which means exactly the same thing). You might encounter it in older books, like when a character in a classic novel goes out of doors for a walk. Today, most people just say “outside” or “outdoors,” but out of doors still appears in writing, especially when someone wants to create a slightly formal or traditional tone.