JFYI is an acronym standing for "just for your information". You can think of it as being a kind of preemptive response to the question "Why are you telling me?". For example, in the exchange:

"I'm not going to be home tonight."

"Why are you telling me?"

"Just for your information."

Your paraphrase of "simply for your information" captures this better than "only for your information" (which might be ambiguous with another reading whereby you mean something like "for your eyes only").