Openly having multiple (sexually intimate) relationships.
It can take many forms: swinging, friends with benefits, free love, open relationships, polygyny, polygamy, etc. The primary feature is that all participants openly consent to the non-monogamous arrangement.
Polyamory sees people as shared assets. The fundamental scarcity of individuals is their limited time, attention or focus. Everything else is an artificial scarcity. You can share possession of these things not ownership of a person.
Similar to contracts for various shared assets, polyamorists develop lists of rules that govern the relationship. Its bounds as well as its commitments are consciously arranged. Polyamorist often use scheduling software to arrange time and availability.
Polyamory can be conceptualized as either a form of "Res privatae" or "Res communes" type propertarian arrangement. How you conceptualize it overall matters less than the specific rules of the arrangement.
Polyamory's essential difference with monogamy is its sexual non-exclusivity. Monogamous people already have multiple relationships of varying intimacy levels, just not sexual ones. Polyamory sees that as an artificial scarcity.
Polyamory, and other open forms of non-monogamy, are merely a type of relationship preference. Monogamous relationships that are similarly openly consented to, are an equally valid preference. Both are compatible with P2P values.