One of eBay's categories for postcards on the USA site is "patriotic", and this certainly fits in that category. This is a standard/sized postcard of a 45 star American Flag, postmarked in Boston, Massachusetts in 1906. The waves in the flag are embossed, giving it both physical and visual depth. This card also has an undivided back, meaning it was created at a time only the address (no messages) were allowed on back. The back of this one has the postmark and an address, but no other information. No indication of who sent it.
There is a very faded poem written above the flag, and some other information written at the bottom of the card.
It was sent to an address in Greenland, New Hampshire. There's still a post office there, I looked it up.