The rate distortion (RD) curve is a basic tool for quality evaluation. It looks like a curve line, which shows metric dependence on the bitrate. For the RD curve, any metric can be applied.
In the video coding industry, the RD curve represents the encoder/transcoder performance: the higher the metric value for the lower bitrate, the better.
RD curve is a valuable tool for encoder developers. It helps to compare different versions of their product (to see the progress) and shows how implemented new features affect encoder performance and quality. We can say that RD curves help to determine which features to use, and which ones to discard.
RD curves are easy to read: if we fix the bitrate value, we can see which curve has a bigger metric value on the same bitrate. And vice versa: if we fix metric value, we see bitrate savings for different curves on the same quality level.