You've given too little context. Just is such a common word and can imply so many things...
Let's imagine a situation. You are going abroad and you are worried abt your visa. Your uncle who is a big shot at the Ministry of Foreign Affairs says to you,- Just let me know if any problem arises.
It would imply you needn't take any steps yourself, you should only inform him and he'll see to it.
Student or Learner