If you see a grey shape appear on the screen while using the RateIt app (like the above screenshot), you may have accidentally drawn this line or shape on the screen when enabling Guided Access mode on the iPad. Here's what you can do to fix it:

1. While the RateIt app is on the screen, triple-click the home button on the iPad and enter your Guided Access Pin. The screen should then look like this:

2. Notice in the image above that the shape is surrounded by few dots or circles. Tap the circle that has an 'x' inside it (indication of 'close') to permanently remove the shape.

3. Once removed, tap 'Resume' in the upper right corner of the screen to start the app again.