HyperCard’s windows use the standard Macintosh document style window—like the window you now see on screen before you.
The Next Window menu item (added under the Go menu) cycles through the windows in HyperCard’s document layer, including card windows, script editing windows, and picture windows, by sending the frontmost document to the back.
If you hold down the shift key while selecting Next Window, the rearmost window will come to the front.