Hey Alvan Zaputra!

1. The line you mentioned means, that the element should have an offset to the left by half of the screen width. So when the screen of the device is 300 points wide, the element has an offset to the left of 150 points.

2. How to deal with different devices (make it responsive)? Thats simple. Just use as much Flexbox as possible. It makes the layout automatically responsive for different device widths. Use margins and paddings sparingly.