If I don't understand the conceptual model of the product, I will always be confused, no matter how well-labeled the icons are. For example, imagine a product that has a "New Configuration" command. What is a "Configuration"? Is it like a template that lets me define other objects? Is it a way to rapidly switch between different sets of options? Is it a container, like a folder?
The UI must be designed so that the user can infer the conceptual model just from exploring the interface. This is not always easy.