When content (image, text box, shape, ect) is highlighted, you can right or control click to get a contextual menu.
The Arrange a submenu will allow you to move the text box above or below other objects. Helpful if you want text to appear on top of or overlaid on an image or shape.
Bring to front will bring the highlighted content to the most top position and will cover up anything that's under it.
Bring Forward will bring the highlighted content one layer in front of what it was on top of. One layer up.
Send backward will send the highlighted content to the most bottom position and will be covered by anything on top of it
Send to back will send the highlighted content back one layer from where it was. One layer back.