Febrile Seizures are a type of fit triggered by the high body temperature of a fever. They mostly affect children, up to 5 years old. It's estimated that up to 5-10% of children may be affected at some stage.
The fits tend to be short - in the region of 5 minutes (though Complex Febrile Seizures can last up to 30 minutes.) They usually do not cause permanent damage. To be classed as Febrile Seizures there must be no infection of the central nervous system (which can also cause fits).
The cause is unknown. There are suggestions that the chances of seizures are increased due to the immaturity of the central nervous system. There are also suggestions that the fever (somehow) causes toxic neuro-chemicals to build up in the brain.
There is evidence of a genetic aspect, as the seizures do tend to run in families.
For technical details see : Disease-a-Month Volume 63, Issue 1, January 2017, Pages 5-23