Seers are faeries that have the ability to predict the future with visions and images. Seers are very rare, as the only known faerie with this ability is Elain Archeron.
Seers can see visions that help them predict the future.
Throughout the beginning of ACOWAR, Elain is described as seeming detached. With her new transition to becoming High Fae as well as her failed engagement, her sisters Feyre and Nesta Archeron assume it is just her response to experiencing such trauma and loss.
She says that she often feels that she is dreaming and will randomly speak in what appear to be riddles. This odd behavior matched with Elain's transition to becoming Fae lead Feyre and Nesta to believe that she may have gone crazy.
It is not until Azriel realizes that she is a seer that Elain's riddles begin to make sense. Her eyes are even said to clear once he figures it out as if the Inner Circle's understanding of her power has helped her return to a more lucid state.
She also predicts Cassian's injuries during the Battle of Hybern where he and Nesta are moments from death. It is assumed that this knowledge prompted Elain to drive Truth-Teller through the King of Hybern's neck to save him and her sister.
Elain's powers are not seen or referenced much in ACOFAS, so much so that Feyre wonders if Elain still is a seer or if her power was destroyed along with the Cauldron. However, while Elain and Feyre are discussing Nesta's unhealthy coping mechanisms, Elain asks Feyre if she is referring to her as a seer or Nesta's sister. This shows that Elain still considers herself a seer, implying that her power is still intact despite the Cauldron's destruction.