In many school systems F marks a failing grade, so dream books read it as fear of failure or judgement. It also opens words like family and friend, and the personal association counts most.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.