You'd have to look a bit more carefully, but that appears (based on what we can see so far and typical stove practices) to be an air-heating passage (heatilator) entry, with the air-heating output on the top front louvers, and would be entirely separate from the combustion chamber if so. Connecting a fan allows extracting more heat from the stove, if that's what it is. Connecting a duct allows getting air from somewhere cooler in the house to heat here, if desired.
Ash cleanout for a stove as shown is via the front door that opens (probably the low one with the little spring handle - we can see the back of its housing above this opening in the rear picture), so it's not that.
Without a fan added, air will flow through the heating passage by convection alone. This is not a problem. It doesn't need fixed, fear is not called for, if that's what this is.
I find am "OEM Blower Kit" (no endorsement of supplier or product implied) that claims to fit:
- Regency F1000 Wood Stove
- Regency F1100 Wood Stove
- Regency F2100M Wood
- Regency F2400 Wood Stove
- Regency F2400M Wood Stove
F2450 Wood Stove
- Regency F3100 Wood Stove
- Regency H2100 Wood Stove
- Regency 5100 Wood Stove
- Regency S2400M Wood Stove
"And possibly other Regency models" which is a strong signal that this is probably what that opening is about. But still, look a lot more closely.
There should be a nameplate with the model number on the stove, from which you should be able to find the stove manuals so you can know for sure what is what and what recommended safe operating practices are for this stove.