Final answer:
In InDesign, an upside-down L shaped bracket is a handle for resizing text or graphics frames. It allows users to adjust the dimensions of the frame by clicking and dragging. This is rooted in traditional printing layout methods.
Step-by-step explanation:
In InDesign, a bracket that looks like an upside-down L is typically known as a frame or text frame handle. These handles are present on the corners and sides of a text or graphics frame, and are used to adjust the size and proportions of the frame. When you see this type of bracket, it indicates that you can click and drag to resize the object it is attached to. The concept dates back to traditional printing terminology, where brackets and/or braces were used in print layouts for organizing content.