What is i915.modeset=1 for?

You are using whats called Kernel Mode Setting (KMS) to make sure that your Intel graphic drivers are loaded early in the boot process, therefore making the "fancy" boot screen display correctly.

Kernel mode-setting (KMS) shifts responsibility for selecting and setting up the graphics mode from X.org to the kernel. When X.org is started, it then detects and uses the mode without any further mode changes. This promises to make booting faster, more graphical, and less flickery https://askubuntu.com/questions/1080/what-is-kernel-mode-setting

Also see https://wiki.archlinux.org/index.php/Kernel_Mode_Setting#Early_KMS_start