The words outdoor and indoor can be used to describe the means in which an activity is done. Outdoor could mean carrying out an activity in an open space, while indoor is the opposite of it, i.e, doing an activity inside of a building or in privacy. But these meanings can be applied differently in different scenarios, though not far from its original meaning. Whenever I come across these two words, it sounds more like "In and out" and that seems to explain it easily.
"In" connotes a confinement, to be within and the "door" describes an entrance. So "in-door" says it's within the room and this can range from what is done inside to the decoration that brings beauty to the inside of it, like an interior design. And "out" connotes an open space, to be done in public. So out-door is whatever that is done in public with no confinement or secrecy.
But outside of these, outdoor can also refer to an African tradition where a child is publicly announced after being named. The act of presenting such a child publicly is termed outdoor. Also, the concept of outdoor is broad, it can be used in an organization where a support or charity comes from outside or away from that organization. Inasmuch as the concept of outdoor and indoor can be broad, it defined the two terms "inside and outside". This can be easily understood by anyone. Whatever is done in or inside of a house or an organization is indoor and whatever is done outside of a house or comes from outside of an organization is outdoor.
