[drunkenmaster]

This might sound silly and the answer is probably so obvious but can somebody tell me what a shutter actuation is?

Many thanks

[Crocodile]

Simply the number of times that the shutter has been fired (shots taken).

[Brammers]

When you take a shot the mirror swings out the way, then the shutter comes into play. If you want a look at how the whole process works then set your camera to 2 second mirror lockup mode in S mode with about 5 seconds exposure time and take a picture while the lens is off. You'll see the mirror swing up and out of the way, revealing a black sheet of metal at the rear of the chamber - the shutter. After 2 seconds this will flick open and reveal a shiny sheet of glass - that's the sensor and its protective covering. When the exposure is complete the shutter will close and the mirror will flip down.

Be aware that doing that will expose your sensor to dust - don't do it often! You should get away with once, and you're going to have to clean your sensor anyway at some point.

Crocodile is correct in stating that the number of actuations is simply how many times this has happened.