externalized string()?

what does it mean exactly?

Well, did you read the description right under it?

yes, and i have no idea what that means, care to enlighten me?

In the editor you have the options to export/import strings.
The strings are saved in a file with the extension wts.
I have noticed that some strings aren't saved in this file so I suppose that using this fonction will force the editor to export the given string.
war3map.wts is a file inside the map also containing the strings from the map.
I am not sure of this if someone could confirm or invalidate.

It is also very useful for another situation: when the interface allows you to only type a single line for a string in a dialog box (such as when using the Set Variable action). Simply use Externalized String, and you can now enter multi-line text. :D

What Telperion01 said is 100% accurate.

About the war3map.wts file. That file does contain eevry string ever typed in, which includes map description, title, subtitle, name, custom unit names, custom hero names (and thier proper names) item descriptions, unit desctiptions, basicly anything you type in, if it doesnt get used right in the .j file, its thrown in the .wts

is number data stored in this as whell like z height of an hill?

no, only text strings, hence the name *.wts = Warcraft Text Strings