The active representation of an assemblyDefinition node is the one that currently displays in the scene to represent the production asset.
You can change which assembly representation is active depending on the performance requirements of the task at hand. For example, you might want only locator representations active while you block out the scene, but you'll need the Scene representations later as you decide how to light a character for rendering.
(The available representations appear at the top of the Scene Assembly menu. The active representation displays in bold.
The selected representation is now active and displays in the scene.