In the context of personal injury or tort law what is foreseeable is key. It refers to something that is known or reasonably expected. For instance, if water is left on the floor in a grocery store it is reasonably foreseeable that a customer will slip on that water. That foreseeable behavior is the basis for liability of the store.
Foreseeable Behavior Examples
If a building owner leaves a stairway with part of a tread missing it is reasonably foreseeable someone will fall on it. Likewise, if a doctor comes into an operating room under the influence of alcohol and performs a procedure it is reasonably foreseeable he will perform in a subpar fashion. This may cause injury to the patient.
If a manufacturer makes a dangerous product for household use and does not install a child proof protector then the conduct of a child opening and swallowing the substance is foreseeable. That may be the basis for liability of the manufacturer.
Foreseeable Behavior As To Causation
Foreseeable behavior may tie in with the concept of causation. For instance, look at one of the examples given above. Suppose a manufacturer makes a dangerous household substance without a child proof top. Suppose then that an adult consumes the contents of the container. Is that foreseeable behavior that would impose liability on the manufacturer? Probably not. That is there is no causal connection between the absence of the child proof top and the adult drinking the contents.
For more information on foreseeable behavior and other related personal injury concepts see the other pages on this site. Also see the pages on Wikipedia.