A cost object means a cost or a revenue collector wherein all the costs or revenues are collected for a particular cost object. Examples of this could be cost center, production order, internal order, projects & sales order.
So whenever you look at any controlling function the basic thing you need to ask yourself is What is the cost element(expense) I want to control and what is the cost object ( i.e. either the production order, sales order, internal order) I am using to control this cost element. Sounds confusing read it again it is very simple.