This option is available from the "Insert | HtmlHelp Control" menu and is a HtmlHelp specific option. This option is rarely used because of a quirk in HtmlHelp v1, it can affect how your help file is redrawn, i.e. the screen redraws are patchy. But some use it and it is included for comprehensiveness.
Adding a Splash Screen means that when the Topic you have added it to is opened a bitmap will be displayed for a specified number of seconds.
Splash Screen Tutorial