That is the Color Sampler Tool
You can find it under the Eyedropper Tool, which it behaves exactly like, except that it saves your selection. You can see the sampled colors in the Info panel (you can hit the eydropper dropdown on specific samples to see different readout values). You can save up to 4 different samples in CS6 and I believe up to 10 samples in CC.
To remove all of your samples simply hit the “Clear” button in the top bar (with the Color Sampler tool active). To remove a single sample you can Alt+Click the sample (again, with the Color Sampler tool active).
Source : Link , Question Author : brain56 , Answer Author : Community