More information can be found here.
The section HLCs online shows you how many NUCs are currently online. This can be useful for debugging or to find out on how many NUCs you can remotely deploy your program before, for the rest of your selected vehicles, the computation for those takes place locally. You can expand the section by clicking on the little triangle, thus revealing the IDs of the HLCs which are currently online.
The following section Reboot HLCs allows you to reboot HLCs of your choice. You again need to click on the little triangle. An input will appear:
You can find out what to enter here if you hover over the input. You can either enter a comma-separated list that the LCC is supposed to remotely reboot (you can check if that worked by examining the online HLCs simultaneously), or you can enter a '*' to tell the LCC to reboot all currently connected NUCsMore information can be found here.