As a teacher, I often get this question a lot during class. I'll make it as easy as possible. ---------------------------------------------- กู (goo) = I มึง (mueng) = You ---------------------------------------------- You might have heard of ผม (pom) or คุณ (kun). It means I (for men) and You, respectively. These words are polite.
กู (𝐠𝐨𝐨) & มึง (𝐦𝐮𝐞𝐧𝐠) are, however, impolite.
They're widely used among close friends. People also use these words to address others or themselves when they are very angry or when they want to show disrespect.
As a non-Thai learner, I recommend you to be polite by using คุณ (kun) to address others instead of using มึง (mueng) if you don't want to sound offensive.