If you sue someone, you're accusing them (in court) of doing something wrong or illegal and demanding that they pay for it. Almost every time someone sues, they're looking for money.
One of the last things you want to hear is "I'll sue you!" People sue for a lot of reasons. If you slipped on the ice, you could sue the sidewalk owner for not putting salt down. If you were in a car accident, you could sue the other driver. Years ago, someone sued McDonald's for scalding themselves on a cup of coffee they said was too hot. Sometimes it seems like people will find any reason to sue.