It's the number of times, more or less, a picture was taken. It's more or less, because a shutter can be raised even if a picture is not taken, like when the sensor has to be cleaned. Most camera manufacturers put the shutter life to 100,000 actuations. This does not mean that your camera will die exactly at 100,000. It also doesn't mean that your camera won't die if it does not yet reach 100,000. That 100,000 is a lot even for pros. If you take very good care of your camera, it will last a long time.
You can use shutter actuation count to estimate the age or extent of use of a digital SLR. The higher that count is, the more often the camera has been used.
Now you know!