A raked stage is one that has the back of that stage raised. This borrows one aspect of linear perspective - that of objects that are higher being farther away. In this case it reinforces the depth of those objects or people on that stage. See an example of a raked stage here. It is also the source of stage directions such as upstage and downstage. A character that is downstage is closer to the audience.
Audience seating can also be raked as well, but that is not the concept that is being discussed in this concept.