Google Scholar is a freely accessible database that indexes the full text or metadata of scholarly literature across an array of publishing formats and disciplines. Google Scholar index includes most peer-reviewed online academic journals and books, conference papers, theses and dissertations, preprints, abstracts, technical reports, and other scholarly literature, including court opinions and patents (Wikipedia).
To fetch entries from Google Scholar, choose Search → Web search, and the search interface will appear in the side pane. Select Google Scholar in the dropdown menu. To start a search, enter the words of your query, and press Enter or the Fetch button.
Google scholar can block “automated” crawls which generate too much traffic in a short time. Normally, the results are displayed in the import inspection window. In case an error occurs, it is shown in a popup.
However, after too much crawls JabRef — or more precisely: your IP address — could be blocked. To unblock your IP, doing a Google scholar search in your browser might help. You will be asked to show that you are not a robot (a CAPTCHA challenge). If no CAPTCHA appears, or JabRef is still blocked after performing a search in the browser, you can also change your IP address manually or wait for some hours to get unblocked again.
Thus, the Google Scholar fetcher is not the best way to obtain lots of entries at the same time. If you are using Mozilla Firefox, the JabRef Plugin “JabFox” might be an alternative to download the BibTeX data directly from the browser. You can find the PlugIn here: https://addons.mozilla.org/en-US/firefox/addon/jabfox/?src=external-jabrefSite.