How exactly this setting works depends on the data source:
Google Search Console only lets you fetch data on the last 90 days. With this setting, you can get longer time spans of GSC data. How it works is, when you first run a GSC query with this feature turned on, our tool stores the results on our server. When you later request the same data (same accounts, metrics, dimensions and filters), our tool combines the latest results with any old results stored on the server, and returns the merged results to your spreadsheet. To use the setting, simply check the "Combine new results with old" setting on the Options tab in our sidebar. For this setting to work, you need split the query by a time value (month, week or date). With this setting on, your query results are stored for the long term on the Supermetrics servers, though they are available only to you, and we don't make any use of the data other than for your queries. After you stop accessing the data, it will automatically be deleted from our servers within six months.
Other data sources: in some cases, a query may be very slow to run for a long date range. With this setting, you can set the date range for a shorter duration (e.g., last 2 days), and set the query to refresh every day. Every time the query refreshes, it will only fetch new data for the last two days, but will keep the old data in the sheet. Note that if some dimension combinations are found in both the existing and the new results, then the metric values for those rows will be updated based on the new results. Note that new and updated rows will always be inserted after the existing rows, so any sorting you have specified will not be applied across new and existing rows.
Great! How do I use this feature?: See How Do I Use the 'Combine new results with old" Setting?