Mrs Potiphar is bored! She wants a distraction and Joseph seems to be exactly what she is looking for. He is handsome, intelligent, and trustworthy.
With her husband away, Mrs Potiphar tempts Joseph to sleep with her repeatedly, but he refuses.
Angry and humiliated that a man should reject her, she formulates an evil plan that will get Joseph into hot waters and probably killed.
You can find the biblical account of this story in Genesis Chapter 39.