This is the Olympus Pen F / FT's Light Seals foam (backdoor gasket sponge) replacement kit.
When the original light seals become old, the camera can experience light leaks and backdoor rattling. Replace them with new seals so you don't have to worry about those problems anymore.
Please note: This kit fits Pen F and Pen FT only. It does NOT fit other Pen series cameras.
This seal kit covers the green halftone portions shown below. It seals light from the outside perfectly.
Click to enlarge
Usually you do not have to install this. If you find light leaks, try installing it.
As for the prism seals, you should not install them unless you are experienced.