The normal meaning was explained clearly by previous people, as a Chinese learning Japanese I want to say some other things.
Chinese: it's(Third person possessive pronoun)
Japanese: it(Third person personal pronoun)
X 之 Y:
Chinese: Y of/in/etc X, X's Y ~= 的(Modern)
Japanese: mainly X's Y, == の
We Chinese only use "其一"(Chi' i in Mandarin, Ki' iat in Cantonese) to explain the same meaning as Japanese "其之一"(その一, sono ichi)
Back to the topic, Japanese "其之一" means first of something, something might be a actual topic/person/thing or others. We Chinese & Japanese & Korean usually use Third person pronoun to represent the current topic.