Select an image in your story and click on Add overlay text from the right side panel.
Type directly on the image to add your text.
To adjust the font, color, size etc., highlight the text as explained here.
Moving Overlay Text
Advanced grid/Pro Layout Builder
Using advanced grid or the Pro Layout Builder, you can drag and drop the position of the overlay text on an image.
In autogrid, moving the overlay text will be more restrictive.
You can use the text alignment buttons (left, right, center) and the textbox alignment buttons (top, middle, bottom) to customize where the overlay text sits. You can also use the vertical or horizontal padding textbox options to customize the placement further.
To move overlay on a slider, first convert the grid to autogrid, customize the overlay placement and then convert the grid back to a slider.