Ecto.Query.API.exists

You're seeing just the function exists, go back to Ecto.Query.API module for more information.

Evaluates to true if the provided subquery returns 1 or more rows.

from p in Post,
  as: :post,
  where:
    exists(
      from(
        c in Comment,
        where: parent_as(:post).id == c.post_id and c.replies_count > 5,
        select: 1
      )
    )

This is best used in conjunction with parent_as to correlate the subquery with the parent query to test some condition on related rows in a different table. In the above example the query returns posts which have at least one comment that has more than 5 replies.