It isn't necessary that the reader or listener know all there is to know about an object for you to use "the". It's sufficient that they know which thing you are talking about, in at least some general sense.
If I am talking to you on the phone and you are at a home and you said "I'm sitting at the table", then unless there was some other information, I'd assume you meant your dining table. For most people, the dining table is the only table or the most prominent table in their house, so it's "the table".
If you're in a place where there might be several tables or many tables, you'd normally say "a table". Once you identify a table, it becomes "the table". Like, "I'm sitting at a table in a restaurant. The table is pretty small." It's "a" at first mention, but after that it becomes "the". Note the listener doesn't have to know anything about the table other than "the one I just mentioned" for it to become "the table". He doesn't know what color it is, how big it is, what it's made of, etc. All he knows is, the table I just identified.