When an image is cropped to meet your desired dimension or aspect ratio, some parts of the image will be cropped out. The tricky part is to know which part to crop off and which part to retain. Auto focal detection helps to identify which part of the image is important.
By default, the browser doesn't allow a web page to save multiple files. The first time CRAP tries to save multiple files the browser will alert you. If you accidentally disallowed it, you can change this in your browser's settings page. Alternatively, you can use the save as zip file function.
You can easily find out how to reset your browser by searching keywords like "allow chrome download multiple files" or "allow firefox download multiple files"
C-R-A-P looks for xxxx's in the name and replace them with numbers. For example, my-photo-xxx will rename photos into my-photo-001, my-photo-002, my-photo-003 etc. You can put as many x's as you like.
Generally I recommend people to use 80%. You should never use more than 90% unless you have a good reason.
I assume the images resized are meant for online use. To improve the loading time and save bandwidth for mobile phone users, you should try to keep your images as small as possible. Of cause the photos shouldn't be so pixelated and they affect the aesthetic of your website. You can try out different qualities and find the optimal number.
Pixelation happends when you up-size your image and there isn't a good way to avoid this.