Final answer:
The question is about a user interface element known as a button, which is an interactive rectangular object that changes appearance when clicked to indicate activation.
Step-by-step explanation:
The subject of the question pertains to an interactive element commonly found in graphical user interfaces. Specifically, the rectangular object you can click, which changes appearance to look pressed, is known as a button. This element is often used to initiate actions or commands within a program or application. For example, clicking a button on a website can submit form data, or in an application, it might open a new window or save changes. The appearance change when clicked provides visual feedback to the user, indicating that the button has been activated or pressed. This feedback is a crucial aspect of user interface design, enhancing the user's interaction with the software.