An empress is either a female emporer or the wife of an emporer. Either way, it's a powerful woman.
There are a lot of words that have gender built into them, like fireman and waitress. Another is empress, which is a female emperor. An emperor is like a king: a person with absolute power over a country. An empress is therefore like a queen. Also, just as a queen is the wife of a king, an empress might be the wife of an emperor, meaning she's not the ruler of the country but just married to the ruler.