'Asshole' is a bad word, or a 'cussing' word. Such as 'f%ck'
It means: rude, disrespectful, hateful, stupid. It's a really strong word and it referes to 'butt holes or anuses'
'I was working for assholes' means that person was working for very disrespectful - rude people that made her depressed or upset.