A seizure is a problem caused by a sudden burst of electrical activity in the brain. With a focal seizure, the burst of electrical activity stays in one part of the brain. Therefore, you tend to have localised (focal) seizure symptoms. Different parts of the brain control different functions and so symptoms depend on which part of the brain is affected.
This is a companion discussion topic for the original article at https://patient.info/brain-nerves/epilepsy-and-seizures/epilepsy-with-focal-seizures