This documentation applies for JabRef 3.7 or later.
SAO/NASA Astrophysics Data System is an online database of over eight million astronomy and physics papers from both peer reviewed and non-peer reviewed sources. Abstracts are available free online for almost all articles, and full scanned articles are available in Graphics Interchange Format (GIF) and Portable Document Format (PDF) for older articles (Wikipedia).
To fetch entries from SAO/NASA Astrophysics Data System, choose Search → Web search, and the search interface will appear in the side pane. Select SAO/NASA Astrophysics Data System in the dropdown menu. To start a search, enter the words of your query, and press Enter or the Fetch button.
Then, the results are displayed in the import inspection window. In case an error occurs, it is shown in a popup.
Apart from fetching entries by using a full search it is also possible to directly create a BibTeX entry using the BibTeX → New Entry dialog. More details can be found here.