Think of the frame device as the picture frame outside your main story, or the bookends on both ends of your main story line.
The frame could be another story line moving concurrently (in the "now" or in the "future") with the main line of your story. It could also be a narrator starting the story (setting the stage) and then ending the story.
Here are two examples.
The Princess Bride
The main story of The Princess Bride is about Buttercup and Westley, pirates, monsters, princes, giants, swordsmen, miracles, and true love.
A frame, though, starts the story. A boy (played by Fred Savage) is home sick from school, and his grandfather (played by Peter Falk) comes by to keep him company and entertained with a story --- so the grandfather reads his grandson The Princess Bride.
That frame reappears in snippets throughout the movie, interspersed with the main story line about Buttercup and Westley. At the end, The Princess Bride has closure (Buttercup and Westley kiss) and then the frame story has its own closure (the boy asks if his grandfather can come back tomorrow and read the book again, and his grandfather answers, "As you wish"). Both story lines are brought full circle.
That's a frame device with its own (simple) story line.
Disney's Tangled begins and ends with a narrative frame device. The character Flynn Rider (voiced by Zachary Levi) opens the story to set the stage (with kind of a miniature prologue of back story and setup) with a voice-over. He concludes the story the same way, in a nod to the traditional fairy tale epilogue ("and they all lived happily ever after ...").
That's a narrative frame device.
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Just for the sake of something different, rewrite the first page or two of your WIP and use a frame device as the opening. It's a little more challenging to use a frame device in a written work and make it seamless, but the frame device is just as legitimate an opening technique as the prologue or in media res.