Exp.SIM stands for "exposure simulation". In Live View, this icon indicates whether or not the image brightness shown on the LCD will accurately portray the brightness of the image as it will be recorded. The appearance of <Exp.SIM> will change to reflect one of three conditions:

<Exp.SIM> is white - The brightness of the image displayed in the LCD is close to what the actual recorded image will look like.

<Exp.SIM> is white and blinking - The LCD's Live View image is not being displayed at suitable brightness due to low or bright light conditions. The recorded image will reflect the exposure setting and not what is being displayed on the LCD.

<Exp.SIM> is grayed- out - Evaluation of the accuracy of the LCD's image brightness is not available. This occurs when the flash is used or Bulb exposure is set.