An active structure is realised - it has existence (the elements of the structure have existential control, so they actually may not exist in the current state of the structure, but that is another story). It doesn't matter where you start in the structure, the structure has the same meaning. Being realised, the structure can be visualised - it can be shown to a user, and the user can see states flowing in the structure, even it building itself.
The stream of instructions for an algorithm has a structure, but it is virtual, requiring something outside itself to give it a sequential structure, by reading the instructions and jumping to where they indicate. It is crucial to know where the instructions started, otherwise whatever is reading them cannot be assured it is in the right state.
A virtual structure is very difficult to build upon, a realised structure is easy to build upon, because the connection points are "right there".