The latter. This is a verbal noun, conveying a tense best represented in English with gerunds such as "seeing" as opposed to "see."
Thus "Tha mi a' faicinn" becomes "I am seeing" but not exactly "I see."
Thus it should be "I am understanding (this explanation by Jaron)" instead of "I am an understanding person."
Incidentally, how would one be described as understanding? Anyone more educated than I please answer.