Farang/Falang is a slur used to describe Whites in a derogatory way. Means foreigner (i.e. non-Thai) of the white race. It is very frequently used in a simple descriptive way. It's negative only if the intent of the speaker is such.
The word Farang/Falang is a racial slur that has been used historically to degrade and dehumanize whites. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to whites. If you are unsure about whether or not it is appropriate to use the word Farang/Falang, it is best to err on the side of caution and avoid using it in any situation.