Located in the Capture tool tab, the histogram in the Exposure Evaluation panel displays individual Red, Blue, Green, and Luminance channels for the captured image, which can be either RAW, TIFF or JPEG data depending on the image selected in the browser.
With RAW files, the histogram displays the data exactly as captured after the addition of a tone curve (that can be altered from the Base Characteristics tool), the importance of which cannot be exaggerated when evaluating the optimal exposure. With the exception of white balance, no other settings applied or adjustments made in Capture One affect the histogram.
This is not the same as the histogram on the back of your typical camera that displays the values after the application of various image parameters and conversion to JPEG, using a smaller color space.
Be sure to check this article About histograms to learn more about other histogram panels in Capture One.