The  in the 'uci' command refers to the index of the configuration entry in the router 'ping_reboot' section. It means that you are setting the values for the configuration entry with the index of 10 in the 'ping_reboot' section.
It's important to note that the index '' is arbitrary and you can use any valid index number for your configuration entries. If you don't specify an index, 'uci' will automatically assign one for you.
When adding a new 'ping_reboot' instance, you should use an index number that is not currently being used by any other instance.
In your case, when you used the '[-1]' index with the 'uci' command, the uci tool will automatically assign a new valid index number for the configuration entry being added.