If you filed a trademark application in a country outside the USA within the last 6 months, and then file a USA application for the same mark (trademark and goods/services are the same), you may be able to backdate your USA priority date to the date of your non-USA filing. This is called alleging a 44(d) priority basis. Not all foreign filings qualify, so it’s important to discuss your particular facts with an attorney.
When can back-dating your use filing priority date be useful? Imagine if one week before you filed your USA trademark application, someone else filed an application for a similar mark covering similar goods/services to you. If you could rely on an earlier foreign filing to back-date the priority date of your USA trademark filing, you could actually be the one with prior rights.