The functionality of the LED lights is described in paragraph 3.10 on page 62 in Technical Product Specification https://www.intel.com/content/dam/support/us/en/documents/mini-pcs/nuc-kits/NUC8i7HVK_TechProdSpec.p... . The third from left LED indicates Ethernet activity and the fourth LED indicates system performance (based on power consumtion).
Adding to Leon's answer, it is also possible for you to assign different usages - and indeed different colors - to these LEDs. This is done with the Intel NUC LED Manager, which you can download from here: https://downloadcenter.intel.com/download/27641/NUCs-LED-Manager-for-Intel-NUC?product=126143. Links for downloading the LED Manager User's Manual and LED Manager Release Notes. Links for downloading these documents can also be found on this download page.
Hope this helps,