Hello Carolann,
Image stacking is a collage, a montage of separate pictures into one.
The most frequently used techniques are HDR (stacking) and Pano (stitching).
HDR
HDR stands for High Dynamic Range. It will compose 2 and up to 9 pictures, or version
of a same subject at different luminosity, into one. The part of each picture that is selected
given certain criterea will be kept and added to the other parts to compose a new view of the
whole photograph.
The word stacking is used because the picture fragments are added on top of one another.
Pano.
Pano stands for panorama: extended views that may contain many frames of the camera
that were "stitched" together side by side.
A quick search will give you plenty of videos for both techniques. There are others, but let's
start with these.
Have a good time!