A polyfidelity relationship means, in simple terms, that it is not a monogamous relationship, but that those within the group are faithful to each other. Poly is latin for many, and fidelity means faithful. So, it is a relationship of many people. All of those within this kind of relationship enter into it with consent.
More About Polyfidelity
In relationships with more than two individuals, each member of the group commits to maintaining sexual faithfulness to the group. Generally, no one else is brought in without first discussing it with the group.