The ImplicitResourceKey.KeyPrefix property ties together a collection of properties that can be assigned to a control. The ImplicitResourceKey.KeyPrefix property refers to the value after the colon (:) in an expression, if present, and before the first period (.). For example, suppose you had an implicit resource in the following form:

<asp:label runat="server" id="label1" meta:resourcekey="Welcome"
        text="Welcome" 
        ie:text="Welcome Internet Explorer User"  
        forecolor="blue"/>  

This resource contains three keys, Welcome.Text, Ie:Welcome.Text, and Welcome.Forecolor, that potentially have localized resource values to retrieve from the resource file or source code. Each key has "Welcome" as the ImplicitResourceKey.KeyPrefix value.