LOOKUP - now that you have data in your spreadsheet, you may want to compare it with other data from a different data source.
Similar as with import, select your data source and filter accordingly. Make sure you select at least one property that matches with the existing spreadsheet, e.g. Date, so that the new data will be matched to your existing data. On the next page, you will be prompted to indicate which properties you want matched.
Voilà, your new data is imported and matches the existing data according to the property you selected.
Here's a video showing how you can use Lookup to compare ad metrics from Facebook and Google Analytics: