Febrile is an adjective that means "related to fever." It can be used in a medical sense when someone is sick and running a temperature, or in a figurative sense to describe a state of excitement or energy.
When febrile is used to describe a fever due to illness, it is often used together with the word seizure. A febrile seizure is a seizure triggered by a fever. In a non-medical sense, the word can describe a state of excitement, as in, "The atmosphere in the city was febrile as the king's coronation date approached." You might think the fe in the word sounds like "fee," but it is actually pronounced "feh," as in February.