Nature Renderer loads the details for your terrain at runtime while you move around the terrain, only keeping the details within range loaded. This helps to keep the memory usage low.
Streaming is done on a separate worker thread and is usually not noticeable. The distance at which the details are loaded and discarded can be set in the 'streaming' section of the Nature Renderer component. These properties can be kept at the default values in most cases, unless you specifically run into streaming issues.
Stream in Distance
The distance at which to start loading terrain details. This distance is added to the Detail Rendering Distance
Stream out Distance
The distance at which to discard loaded terrain details. This distance is added to the Detail Rendering Distance.
Stream Processor Limit
The number of CPU cores to use for streaming. This number is multiplied by the total number of cores on the device.