It appears this question maybe coming from UK as earth leakage is not commonly used term here in US. What we refer to as "ground" - they refer to as "earth"... technically one and the same thing. Earth leakage is when either live or neutral is leaking to earth through a bad insulator.

Gene.

Here is a good site with a lot of electrical info
Electric system grounding inspection, diagnosis, & repair guide

does this mean there is a short to earth?
Could be but not necessarily.
Could be that just part of the current is leaking to ground. This can be caused by moisture or conductive dirt.

Basically this is how a GFI works.
It monitors the current on the Line and also the current on the Neutral.
If they are different it trips (Line higher than Neutral).
Note: this can be milliamps of difference.
The amperage difference has to be going to earth ground.

That could be to your ground connection.
OR
Through moisture or dirt to the case/frame
OR
Through you. If the unit's case has gone live, you are touching it and you are also grounded to earth.

That is why most appliances use a 3 wire hook up because if you get a dead short from live to a ground, it should blow the breaker.