The Ha is the cutting edge of the Katana. This part of the blade is differentially hardened and will therefore have a much higher rigidity and hardness than the rest of the blade. The HRC is also different from the rest of the blade.
It is important to have a perfect mix between hardness and sharpness, because a blade that is too sharp will be very thin and could get dull or even break.
Many beginners in the field think that a razor sharp and very fine edge is absolutely necessary to cut. However, a Niku edge (visually less fine) is even more efficient, because it is more resistant and able to go through more solid materials, even if it doesn't look like it.
How to sharpen a Katana
If you want to learn more about sharpening a blade to resharpen your Katana or just for your knowledge, here is a video that shows how to do it !
You can find stones for sharpening a Katana on the internet quite easily.