When you draw walls/construction lines, they can be one-sided or two-sided.
By default, walls and construction lines are one-sided unless this setting was changed in the wall/construction line preferences. The preference then applies to all future walls drawn.
For instructions on changing an already drawn wall setting, see Lengthen or shorten a wall at the beginning or end.
A one-sided wall or construction line offers only one placement zone, inside the wall, where you can place items.
A two-sided wall or construction line offers two placement zones, inside and outside, where you can place items.