ScriptInfo.xml is a file placed in the script's directory (along with ".js" files). It is read when the script is loaded or imported and contains several information related to the script such as its name, its description, the menus it displays in Messenger, etc... the file does not need to be created for scripts to run locally, however, its presence is mandatory if the script has to be exported into a Script Pack.
The Schema file for ScriptInfo can be temporarily placed in your script's directory to validate your XML. However, the schema file must not be redistributed with your script (it would only be dead weight anyway). Here is how to write the first element of your XML file to make your XML editor aware of the schema file:
<ScriptInfo xmlns="urn:msgplus:scripts" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:msgplus:scripts PlusScripts.xsd"> <Information>...</Information> </ScriptInfo>
It is good practice to always link your xml files to their respective schemas to prevent any possible run time error caused by an error in your XML. XML files tend to grow rapidly and you'll soon be thankful for the validation layer brought by the schema file.
If your XML Editor does not come with graphical browsing of the schema file, you can view a documentation generated by XMLSpy in the ScriptInfo Schema Documentation (some information may be missing in this documentation). Some elements of the file are also explained in details in Packaging your Script.
ScriptInfo Schema File, Schema Documentation, ScriptInfo Examples.