Some laptops will have inverters to convert DC voltage into AC voltage. For example, displays that use CCFL backlights will need an inverter since CCFL uses AC voltage. Remember that an LED display uses DC voltage, and therefore would not need an inverter.
The CompTIA A+ 220-1001 objectives want you to be able to install an inverter in a laptop display.
Typically inverters in laptops are long, narrow circuit boards located below the LCD panel and are attached via screws or adhesive tape. Below is a sample of what an inverter looks like:
The inverter should have a connection to the power supply and a connection going to the CCFL backlight.
Indicators of a faulty inverter include a black or dim LCD display. If the laptop can still run an external display, then the integrated GPU is probably fine and so you might need to check on the inverter.
Below is a video demo of replacing an inverter on a laptop:
