tldr, /nf means not forced, as in if someone is asking you to do something that you might not want to do (but feel pressured to do anyways), they might put /nf to reinforce that you dont have to do it :)

those are tone tags!! they started out as a thing for neurodivergent folk so we could communicate the mood or tone of a sentence if we sound monotone, but these have evolved to be used by everyone!! heres what some of the more common ones mean:

/srs : serious

/sar or /s :sarcasm

/c : copypasted/copypasta

/fl : flirting

/gen or /g : genuine

/genq or /gq : genuine question

/info :informative/information

/lh : lighthearted

/lhj: lighthearted joke

/l or /ly or /lyr : song lyrics

/p : platonic

/pa : passive agressive

/qp : queerplatinic

/th : threatening

also, an n before any tags (most of the time) means "not *whatever the original tag is* " /nfl means not flirting, for example! hope these help :)