This windows gives access to all the viewer settings. Many of these are already accessible via the Preferences window or the top menu bar. Quite a few others, however, are not.
NOTE: Do not change settings here unless you know what you are doing, or are requested to do so by a member of the Support Team.
A full list of debug settings is available here.
To change a setting:
Type in the setting name in the drop-down. You will notice that it attempts to auto-complete the name as you type.
On versions later than 3.2.2, it will match even if you type something other than the start of the name. For example, render will match (among other things) DebugShowRenderInfo.
Once you have the correct setting, set the value you want below. This might be a True/False type setting, or it might want a number; it will vary depending on the specific setting.