GF help maintaining threat

Hey I just started playin maybe a little less than a month ago, and I got my GF to 70 and I’m having a hard time maintaining threat/dealing damage. I’ve done some reading and I came across an RI cap I think it’s called which ties into my ArPen. And needing it to be at 40% or 60% but I’m not exactly sure what these percentages relate to, or how to effectively raise ArPen outside of enchantments.

Could anybody give me a brief run down of the ArPen/RI cap thing? And give me a few tips on maintaining the threat ?