If a building owner leaves a partition in an unstable fashion knowing that people are frequently walking through the hallway where that partition exists, then it is reasonably foreseeable that someone may touch that partition causing it to fall and producing injury. That is foreseeable behavior.
Likewise, if a physician comes into an operating room under the influence of alcohol and performs a procedure on a patient it is reasonably foreseeable that the physician will perform the procedure in a subpar fashion causing injury to the patient. The negligence of the physician in that circumstance is foreseeable behavior.
If a manufacturer makes a dangerous product for household use and does not install a child proof cap then the conduct of a child opening and swallowing the substance is foreseeable behavior. That may be the basis for liability of the manufacturer.
Foreseeable behavior frequently ties 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 consuming the contents.