A key term that appears early and often is 'avidya' or 'ignorance as is the usual translation. What does it mean in the context of Advaita? Ignorance is something that is displayed and the central paradigm case of that is the taking of something to be that which it is not. In other words a mistake displays your ignorance. The real nature of what we thought we knew escaped us. Now one might say that this is a very narrow definition of 'ignorance' almost amounting to a technical use which is correct because of course there are 'unknown unknowns' which we are not in the slightest danger of confusing with anything else. They do not exist for us.
The analysis of 'avidya' follows through to the manner in which is displayed. This is known as 'adhyasa' or 'superimposition' and here we get to the classic example of the snake/rope. The attributes of a snake i.e. being coiled, are projected/superimposed on to the harmless rope.