Mainstream and substream are two different video streams used to view your camera footage.
Mainstream: is the primary video feed. Mainstream provides the highest video quality, and is the stream the DVR / NVR uses when saving footage to the internal hard disk drive (HDD). Mainstream settings can be adjusted to increase or decrease recording resolution. These settings will impact recording file size and available recording time.
Substream: is the secondary video feed. Substream provides a lower quality video stream, and is typically used to stream video to computers, smartphones, or tablets over the Internet. This allows you to reduce streaming quality for live or remote viewing, while maintaining higher quality on the mainstream for HDD recording and storage.
For more information on how to adjust the mainstream and substream settings for your specific DVR / NVR refer to your user manual. Product downloads and documentation