When someone uses the phrase "the knives are out," they typically mean that people are taking a very critical or hostile approach to a situation. Other ways to express this sentiment might include saying that people are going on the attack, or that they are engaging in cutthroat tactics. Some people might also say that things are getting ugly, or that a conflict is escalating quickly. Regardless of the particular phrasing used, the implication is always that people are taking a very adversarial approach to something, often with little regard for the feelings or interests of others.