If all is going well your debug log will hopefully be empty.
However while developing new modules or skins, it is quite common for mistakes in code to cause the debug log to fire.
When somethings wrong but you don't know what it is, the Debug Log can be a useful tool to help you figure out what is wrong.
Here is an example of an entry in the debug log.
 => xtDJLicense
 => Array
[jrcore_list_function_call_is_active] => 1
[search] => Array
 => _item_id in
 => _profile_id IN (SELECT SQL_SMALL_RESULT `_item_id` FROM jr_jrprofile_item_key WHERE `key` = 'profile_quota_id' AND `value` IN(1,2,3))
[limit] => 6
[template] => xtDJLicense_stems.tpl
[order_by] => Array
[_created] => DESC
[module] => xtDJLicense