Many causes. Foot drop is an inability to elevate the toe, which can cause you to trip. It can be caused by a wide range of diseases, ranging from brain issues to pinched nerves in the back and leg. A neurologist can help to narrow down possible causes and help you establish a plan of treatment.
See below. Dropfoot has various causes but results from the muscles that dorsiflex the ankle (primarily tibialis anterior) not functioning. This usually is caused by a nerve dysfuntion. In some cases it is reversible if the nerve can recover. Therapy, brace and surgery options exist.