In mathematics, what is a Galois field?

if you didn't know this , don't worry, I did several semesters of group theory at university. I didn't know. Galois is attributed with starting group theory, although it was not identified for a long time after his death. He died in a duel over a woman, I think 21 years old (Galois not the woman). He is very famous as one of the many " had he lived, what marvels he would have achieved".