That is the screen prior to booting that allows you to select which disk to boot from e.g. If you have windows installed as well it will display 2 disks to boot from. It should only appear when you hold down the ALT key as the computer starts up.
Make sure that you have a default boot drive set by going into
System Preferences > Startup Disk >
Make sure that the 'Macintosh HD' is highlighted.