What does a midwife mean if she says 'it's at the brim'?

By 'lying in the brim' we mean that the baby's head is just at the top of the bony pelvis (pelvic brim) so the head is not engaged but almost certainly will be soon. As you have only 4 weeks until your due date it sounds as though everything is going in the right direction - that is downwards! sounds to me like she is referring to the head of the baby is crowning, meaning that it is approaching the vaginal opening.