Klofningur ("a split") is the bottom part of a mountain called Klofningsfjall and there happens to be a split where the road goes through. I wouldn't say this is something you have to go out of your way to see, but if you're exploring this remote part of Iceland I do recommend this drive since it really takes you back in time. There is a small place to park and from there you can make a quick walk up the rocks where you'll find a viewing platform/disc with information about the surrounding area in sight.
QUICK NOTES:
What it is: A split up rock with a road going through it
Best season: Any (but road can be tricky / inaccessible during winter)
Best time of day: Any
Nice to know: On top of the rock (east side) is a viewing platform with information about the surrounding area in sight