When people use the term "open" to describe their arrangements, it usually means more of a focus on sex, ie., the freedom to have multiple sex partners. Oftentimes, a couple will be okay with each of them having FWBs and fuck buddies but will only invest emotionally and "allow" loving feelings within their dyad. In an open relationship, sex with other people is more recreational and for pleasure, but not for emotional connection.
It's close to swinging, but swinging usually involves many rules and restrictions, usually as a couple and going to sex/swing clubs, while just being open doesn't have so much of that. Being open is like... having a "hall pass," and the people who are open don't usually expect to participate in their sexual liaisons together, like swingers do.
Polyamory is more about having multiple loving relationships, where the focus is on love or romance as well as sex (or no sex for asexuals). In poly, there is no prohibition against developing emotional attachment and having deeper feelings for any/all of the people with whom one is involved. In fact, it is encouraged.
The above are generalities, and some people define these terms differently, but those are how they are generally understood, for the most part. People can also be poly AND open, or poly AND swingers.