Liability is the quality or state of being legally obligated or accountable; legal responsibility to another or to society, enforceable by civil remedy.
In Plain English:
When a person is liable, it means that he or she is legally and financially responsible for an action, and any resulting damages. If, for example, someone is negligent and causes damage because of his or her negligent behavior, that person then is liable for the cost of those damages.
In the case of a lawsuit, the defendant’s liability is determined by the judge, or judge and jury (if it is a jury trial).