After the boot process when you turn on your Android phone, you will be on its Lock Screen.
It is there by default to prevent unintentional activation of the screen. Basically it shows
a screen with sliding buttons which you need to slide in order to access the home screen.
In the lock screen you will see the date and time. You can also activate or deactivate silent mode. There are apps that can customize your lock screen, these are available in the Android market.