Ecto.Query.API.in

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

Checks if the left-value is included in the right one.

from p in Post, where: p.id in [1, 2, 3]

The right side may either be a list, a literal list or even a column in the database with array type:

from p in Post, where: "elixir" in p.tags

Additionally, the right side may also be a subquery:

from c in Comment, where: c.post_id in subquery(
  from(p in Post, where: p.created_at > ^since)
)