When you select h.264+ or h.265+ the i-frame setting is greyed-out (set / managed automatically - you cannot set or change it).
In other CODECs (h.264/h.265 without the +):
They work to reduce bandwidth by only sending a full image initial / I frame once, then partial frames (P-frames) (containing only the changes in the scene since the initial 'I' frame.
So sending I-frames more often improves the video quality, but uses more bandwidth.
The I-frame interval in these HikVision cameras can be set between 1 and 400.
The lower the number, the smaller the interval between initial/I frames and the better the resultant video.
I never tend to adjust it.
h.265+ seems to work well and is very efficient, saving 70-80% bandwidth/storage over h.264.
BTW there is contextual Help available for all the HikVision Configuration pages (top right of each page, adjacent to Logout).
Sometimes it's truly helpful, other times (as in this case I suspect) it leaves you wondering what they are trying to tell you
I looked for a better and clearer description for you, but the official explanations are pretty heavy-going:
Video codec - Wikipedia
Video coding format - Wikipedia
H.264/MPEG-4 AVC - Wikipedia
Inter frame - Wikipedia