As soon as we have begun the boot process and just prior to handing control to the kernel, a boot splash screen can be displayed. This, as the name suggests, controls the splash screen you may see during the boot process. In CentOS, this defaults to the plymouth theme: rings. Plymouth is the boot splash manager; we can use other themes should we wish. Some of these are installed as standard, while others are included in the standard repositories. Yet, more themes can be found in third-party repositories.
You can, of course, build your own theme. Essentially, a minimal theme is just a wallpaper.
Most of the time during the boot process, you will not see the splash screen unless CentOS is your desktop machine. However, I would recommend still working with plymouth to change the default splash from rings to basic. With the basic theme, we can see the services loading during the boot process rather than the rings that merely show the boot progress...