Basically, an environmental portrait is a portrait of someone in a place that is relevant to that person.
a person in their room. a scientist in a lab. an artist in their studio. an athlete in the workout room/field. a musician in a practice space.
Think about just taking a portrait of someone with a background that tells something about them.
I googled "environmental portrait" and these are pretty decent examples.
The location and how they are dressed tell you who they are.