You can connect text frames so that text flows between them. This is called threading text. Text frames have an in port and out port that are used to connect text frames and also visualize any connections. A red plus in the out port indicates that text doesn’t fit in the frame. This is called overset text.
To thread text:
With the Selection tool, click to select a text frame and then click the out port (or in port) to load a text cursor.
Position the loaded text cursor within an existing text frame and click to thread the text or drag to create a new text frame.
Choose View > Extras > Show Text Threads to see the threads between the frames.