Polyamory is the romantic, emotional, and/or sexual attraction to multiple individuals. This can be through a relationship including three or more people or having a string of other relationships that are knowledgeable to the fact they are one of many and consent to such.
Healthy Polyamory - Harry, Ginny and Hermione all love each other equally.
Healthy Polyamory #2 - Ginny love Harry but not Hermione. Harry also loves Hermione, Ginny consents to Harry seeing Hermione while seeing her.
Unhealthy Polyamory - Ginny loves Harry but not Hermione, she doesn't say anything as she doesn't want to rock the boat of their relationship.
Not Polyamory - Harry has five different girlfriends who don't know about each other and will most likely believe they are the only one. This is not Polyamory, this is cheating.
(This has been written through the opinion of someone who is indeed a Polyamorous person. People see Polyamory differently and it isn't for everyone)