This song serves as an ode to the many guitars that have served McCartney well down the years. The former Beatle said in a YouTube video
that he was inspired to pen the tune by his old Martin that was "propped in the corner" of one of his rooms at home. "I normally just have it there so that if I feel like playing, I'll take it down and play it, or try to write a song, or whatever you do," he explained.
McCartney thought to himself that he hadn't had time to play the guitar lately and it reminded him that when he first acquired guitars back in the days of The Beatles, "they would be like a friend. A confidante." He remembered how in those younger days, "he'd go off into a corner and write a little song and almost tell your troubles to your guitar." So this is, "a love song to the guitar."