https://www.odoo.com/forum/help-1/question/what-do-ir-res-stand-for-in-the-base-module-18806#answer-18810

ir = Information Repository

res = Resource

The Information Repository is used to store data needed by OpenERP to know how to work as an application - to define menus, windows, views, wizards, database tables, etc.

All objects that use ir.* or res.* as model name are base objects of Odoo. These objects are, most of the time, necessary to the good running of Odoo.

For example:

ir.ui.menu : The menu of the user interface

ir.actions : The actions that are available under the 'Other option' of a document (in form view)