A defense is a reason why the Plaintiff (the Bank in a foreclosure lawsuit) should not win their case. You tell the Court your defenses when you answer the summons and complaint. You must then prove your defenses in Court. If you prove your defenses then the plaintiff will lose and you will win the case.
Your home is likely the most valuable asset you own. Beyond its monetary value, your home has incredible sentimental value as it represents your hard work, the place where you have raised your children, and the roof over your family's head. This makes foreclosure a stressful and scary prospect: you are literally facing the possibility of losing your home to a financial institution.