Beur is a slur used to describe Arabs in a derogatory way. Verlan (French slang) for "reube", which is itself verlan for "Arabe". Usually used for children of migrants from Maghreb settled in France in the 60-70's. Not inherently derogatory.
The word Beur is a racial slur that has been used historically to degrade and dehumanize arabs. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to arabs. If you are unsure about whether or not it is appropriate to use the word Beur, it is best to err on the side of caution and avoid using it in any situation.