Elsa was revealed to be the mysterious fifth spirit in Frozen 2, and a Frozen short has since confirmed that she wasn't the first person to carry this mantle. Frozen 2 was the highly-anticipated sequel to the 2013 movie, but while it succeeded at the box office, it fell a little short of audience expectations. A key reason for the response to Frozen II skewing more negative was its convoluted plot and mythology, which introduced four elemental spirits, and a strange voice calling Elsa to the magical glacier of Ahtohallan.
It was on Ahtohallan where it was revealed that Elsa was the fifth spirit, a being who served as the bridge between the magic of the four elemental spirits and the human world. Elsa being the fifth spirit served as the explanation for her magical ice powers and is used to help tie up the narrative of Frozen 2, but it also leaves some lingering questions behind. Frozen II doesn't fully explain the fifth spirit's history or what it actually is, and it was left unclear if Elsa was the first to become the fifth spirit - something that the world had always been waiting for - or if she was the next in a longer line of them.
Since the idea of the fifth spirit already existed in the legends of the Northuldra people, then it was suggested that there had been others before Elsa, but the presentation of it as such a big reveal and lack of clarity around the point meant it was uncertain. Myth: A Frozen Tale, a Frozen short streaming on Disney+, clears things up somewhat. Framed as a story set in the Frozen universe being told to two children, it tells the tale of the origins of the four elemental spirits, and includes the fifth spirit in its story too, which it describes as "the human spirit." Interestingly, the fifth spirit was not originally among the others, but instead was born "over time," suggesting that the other spirits pre-date humanity's existence, and then when they came to be the fifth spirit was created. As the narration notes, "Together they created a dazzling harmony, and our world danced."
The Frozen short doesn't make it clear just who the first fifth spirit was, but given the setting is implied to have been so long ago, then it was someone different to Elsa. The story also sets up some of what happened, as the fifth spirit fell out of its rhythm, causing chaos and a lack of balance, and so the other spirits eventually departed the world. This adds even more weight to Elsa's journey in Frozen 2, as she wasn't just the fifth spirit, but was actually responsible for bringing together the other elemental spirits once more, since presumably there wasn't another fifth spirit between the first and her (though it is possible). It also ties her more closely to the Chosen One narrative, since she's bringing back a sense of balance.
There are still several mysteries surrounding the fifth spirit for now, since the exact nature of how and why they came into being, and what powers they possessed, aren't answered here. Nonetheless, it does shine a little more light on one of Frozen 2's more confusing plot elements. It's possible that Frozen 3's story or, more likely, more shorts will continue to explore and explain the fifth spirit and the mythology of Frozen's world overall, further contextualizing Elsa's place in it. It's arguable that the movies still need to reckon more with what Elsa being the fifth spirit truly means, but knowing more of the harmony she brought and how important her role is, given how wrong it went in the past (and perhaps could do so again), at least gives more understanding while setting up intriguing stories for the future.