Final answer:
In InDesign, a red plus sign in a text frame's corner indicates overset text, which means there's more text than can be displayed within the frame.
Step-by-step explanation:
In Adobe InDesign, a red plus sign at the bottom right-hand corner of a text frame indicates that there is overset text. This means that not all the text is visible within the bounds of the frame - some of it is overflowing and cannot be seen. This situation can be resolved by either resizing the text box or linking the text to another text frame, allowing the remaining content to flow into the new frame.