How to run an entity query

I am very confused by entity queries. How do these work? When I try to run a query such as this it fails validation each time because “_Any” or “representations” is undefined.

Do you run an entity query directly against the apollo server or against your own service’s graphql endpoint?

query ($representations: [_Any!]!) {
_entities(representations: $representations) {
… on User {
id
username
}
}
}