It's a video of Lea Salonga singing "On My Own" from Les Miserables. She had appeared in the show on Broadway, replacing the original actress, but performed the role in the superb 10th anniversary all-star cast production done in 1995 (and shown on PBS). The song became part of her repertoire, and there's video of her performing the song this year at the Lytham Festival in England, 22 years later. What this video does is edit those two performances together. The edit here is really well done, but what also stands out is how impressively close her voice, timbre and performances are two decades apart.
(And a fine voice it is. I saw her at the Mark Taper Forum in the interesting remake of of Rodgers & Hammerstein's Flower Drum Song. I've always love a line in the Los Angeles Times review when describing the plot. Talking about the main male character, the critic wrote, as close as I can recall, "And the plot revolves around the question of whether Wang Ta will fall in love with someone who sings like Lea Salonga.")