To answer the question "what is an earth fault?", we first need to address some more basic questions.
What is a short-circuit ?
A circuit is a path that electricity is designed to flow through. A "shorted circuit" is effectively a situation where electricity stops flowing around the designed path of cables because a lower resistance path is available.
What is an electrical earth (ground) ?
The term of an electrical earth or ground comes from the use of planet earth i.e. the ground, as a location that electricity can be directed towards for safety purposes. Electricity is directed towards the ground (for safety purposes) as large amounts of electricity can be dissipated in the ground without the ground becoming electrically active thus avoiding safety issues like electrocution.
Another object which electricity is being directed towards can be used as (or called) an electrical earth.
Note: While the ground on planet earth can handle large amounts of electricity without becoming electrically active the same can not be said of your house !
What is an earth fault ?
An ‘Earth fault’ (also known as a ground fault, isolation fault or leakage current) is a form of short-circuit fault where the active electricity path is accidentally connected to the ground through a fault in the electrical system.
For solar systems this means that the electricity being produced by your solar panels is no longer travelling towards your inverter through the cables and instead is taking an incorrect path towards an object acting as an earth.
The dangerous thing is that your house can be acting as the object that the electricity is flowing to!
Refer to the Should I be worried of an earth fault? for more information on the cause and risks of an earth fault.