I think the best way to explain what I mean by I love you is "knowing you exist makes me happy". It means I want them to be happy, crave them when they're gone, and they're in a special part of my heart.
I don't think it's much different than in a mono relationship. Just, the "I prefer you to everyone else" becomes "I prefer you guys to everyone else". Although I would say it's a different kind of feeling rather than a "better" one. I love my friends too, in a different way, that doesn't mean I think they're inferior.
I think loving someone is emotional and physical. Your heart beats faster, you're all smiley, you know what that's like I assume?
I think in my case "I love you" also means "I want you to stay a part of my life, and while our relationship might stop at some point, I don't have an end in mind".