Dropping this link here with a brief into :

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Ca II H&K stellar time series

At last the rubber meets the road! We have described in gruesome detail how we extract the spectra from the raw CCD frames, and how we place the Ca II H&K spectra on an absolute intensity scale. In this article, we describe (in technical terms) how we produce the time series plots of activity in the various stars on our program. The data products of this step are some of the fundamental results that come from our program, revealing the long-term activity changes in the Sun and Sun-like stars.

Another link:

H-K Project -- Introduction

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The H-K Project uses a specially-designed instrument to measure the amount of light from active magnetic regions in stars. This light comes from calcium atoms that have lost one electron each. The different wavelengths of light emitted by these atoms were labeled long ago. The "H" and "K" light gave this project its name. This light comes from the upper levels of the Sun near active magnetic regions that we can see, like sunspots. Other stars are too far away to see these features on their surfaces. Studying the relative strength of these two wavelengths of calcium light from distant stars similar to our Sun gives an indirect measure of the amount of surface activity on the stars -- "starspots". Using this method, astronomers have been able to follow cycles similar to the sunspot cycle that has been observed on the Sun for centuries.