When clicked it displays the column help:
At first I thought this was a new APEX 4.0 feature and then I tested it on a 3.2 instance and noticed that the column information button was present as well.
Either way, it's a really nice feature to have since you can provide column information to users.
The following query helps to identify which IR columns have the default "No help available for this page item." message. You may want to change this since it says "... page item." rather than "column"
FROM apex_application_page_ir_col irc, apex_application_pages ap
WHERE irc.application_id = :app_id
AND irc.application_id = ap.application_id
AND irc.page_id = ap.page_id
AND irc.help_text = 'No help available for this page item.' -- Default help text message