What does "stacked" mean in this context?

You take multiple pictures at different focal points and combining together computationally because the depth of field at the magnification is very shallow. The resulting image looks somewhat flat, but highly detailed.

You know how some photos have super blurred backgrounds? The same effect occurs undesirably when you’re trying to do extreme macro photography (close-up photos of small things). The effect would be that face might be sharp but the body would get blurrier the farther away from the focus plane.

So a workaround is to take a lot of photos with the focus plane at different depths. You sweep the focus plane through the scene, snapping a lot of photos as you go. This can be automatic with nice gear.

Then you take all of those photos and combined them digitally, with the algorithm selecting pixels from the photo with the best sharpness in that region. So the photo you see is a combination of many photos.