A book cafe is a relaxing place where you go to read books and eat/drink things you would get at a regular cafe. People also read books there.
This is probably a joke but I'll answer anyway
By 'cheeks' they mean buttocks so by "making them clap" is referring to them being so big/thick that they clap or make noise when you walk or move around or something.
I probably got played lol but whatever
The amount of light that passes through the lens and reaches the digital sensor is called exposure.
So, exposure determines how much light or darkness will get to the sensor, and thus how light or dark the image will be when you take a photo. There are settings that are responsible for this: ISO, aperture, and shutter speed - which together are called "the exposure triangle."
