DVRs that display and record at the full frame rate of 30 fps per channel of video are called “real time.”
Many DVRs do not have the processing capability to record in real-time. They record only intermittent frames coming from the camera. A full frame rate produces the smoothest, clearest picture.
A DVR usually specifies the total frame rate for all channels. If you have a 4 channel DVR with a total frame rate of 120 fps, each channel records at 30 fps which means it can record in real time.