When do you need to press the shutter halfway before you can shoot? Can you give an example ~ ~

The positive solution on the first floor, in layman's terms, is that what you want to shoot is not at the autofocus point, so you need to press half the shutter. Let the focus stay somewhere, and then move the lens so that the clear object deviates from the focus. This technique is often used in news photography! The picture below is an example.