Hello,
The [10] 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 '[10]' 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.
Best regards,
Robert