Been wondering about these for a while. Student pilot working on my PPL. When preflighting my Cessna 172 the other day, I noticed these holes just in front of the elevator. What are they for? The image shows the crease between the elevator (white) and the horizontal stabilizer (red) on the left side of the fuselage.
Those are called "Lightening Holes". The name refers to the weight reduction brought about by removing part of the material—lightening—and should not be confused with anything to do with electrical discharge—lightning.
From the FAA's AMT Airframe Handbook, in Chapter 4, page 4-82:
Lightening holes are cut in rib sections, fuselage frames, and other structural parts to decrease weight. To avoid weakening the member by removal of the material, flanges are often pressed around the holes to strengthen the area from which the material was removed.
The particular part depicted in your photo is the left hand rear spar assembly for the horizontal stabilizer, No. 32 in this figure from the 1975 Cessna 172 Illustrated Parts Catalogue (IPC):
As you can see, these lightening holes are present throughout much of the spar and rib structures in the horizontal stabilizer, as well as the vertical stabilizer, wings, and fuselage.
In addition to the gains in weight reduction, lightening holes also serve as openings to allow the routing of electrical wiring, control cables, hydraulic tubing, and other system components. If you look at the same location on the right side of the horizontal stabilizer, you should see the elevator jack screw passing through one of the lightening holes, in the location marked "C" in the Cessna IPC figure above.
That is one of the horizontal stabiliser spars that holds the plating together. It has holes in it so that it would be strong and still lightweight.