Flash

It's the camera shake / blur warning.

You might have the camera set to an inappropriate 'scene' mode. If you are using 'landscape' or 'snow' for an indoor or night shot for example, the camera will select a slow shutter speed which will increase the chance of blur - hence the warning.

If you keep the selector switch all the way to the left (auto mode) you should be fine in most situations. Only use the 'scene' modes for specific purposes.