Probably most commonly/basically it means that the seed coat wasn't humid enough that the cotyledons could extract themselves. I'd need to know more about your growing medium, etc. to be more specific.
The common term used is "helmet head," which might help in finding threads and posts discussing this. Without knowing more, I'd suggest spraying the seeds with water as soon as you see them hook. Don't spray a lot, rather spray reasonably lightly, but often, until you see at least the green of the cotyledons outside the testa and preferable the split of the cotyledons, as that means the growth tip is safely removed. It won't hurt to "over-water" for this brief couple day period, but do avoid soaking the soil completely and be mindful of how well your soil drains.
Also, planting seeds deeper so the seed coat is kept humid in the soil longer, versus dry in exposed air, may help if the seeds were planted shallow. I've always suspected that seeds that weren't dried properly may contribute to HH's, but that's just a guess.