This tool moves the selected components, via using the Selection tool, to the current streaming level. This makes it possible to move sections of a Landscape into a streaming level so that they will be streamed in and out with that level, optimizing the performance of the Landscape.
When the streaming level is visible, the Landscape renders all the components:
You can toggle the visibility of the level by clicking the eye icon () for that level in the Levels window (accessible on the Level Editor's Window menu). Doing so causes the components in that level to no longer be rendered: