What is geolocation parameter / GL code?

TheGLcode shows clickers results "as if" they were in that country. For example, if you wanted to see what the results would be for one of your keywords for someone searching in Germany you could useGL=DEto see those results.

It basically tells google to ignore the user's location and show results as if the user were in the country you specify. Here's an article explaining:

If you are targeting US clickers and you have set the geographic tier to US clickers only then theGLcode is not necessary.