To me, it means more than just what it is. It means I admire, trust, believe in you. It means you are beautiful. It means that I want you in my life forever (but not necessarily in the married sense).
During college orientation at the beginning of my freshman year, my floor started playing this game where you said "I love you." to someone, trying to make them laugh. At first I was okay with it, but after a while, I couldn't participate because I didn't like using the phrase so lightly. It's not because I have been in a serious relationship that fell apart or because my parents didn't love each other. I have never been in either of those situations. I think it's because I grew up in such a stable, loving home. My parents have been happily married for over 30 years, I have never seen or heard them fight. They are equals in the relationship, and they love each other exactly the way they are, mostly. I know there are a couple of quirks one doesn't like about the other, but they're tolerable quirks. Some are even changeable quirks--something that was present in the early years of their marriage, but are no longer present. Things like my dad not liking onions (my mom fixed that early on) or some other equally minor oddity of their preferences or personality. Mostly I know things that my mom used to not like about my dad, and not so much my dad about my mom, because my mom and I talk way more than my dad and I. But that's beside the point.
But because I grew up in a home where my parents were truly in love, I have a very powerful image of what love is in my head. It's a relationship formed on appreciation for the other person, for their strengths and for their weaknesses and for their oddities. It's a relationship based in shared beliefs and shared loves. It's a relationship where both parties are equal, there are no dominant and submissive positions. It's a relationship that takes into account things that the other person likes or does not like, and it does not force a person to change through ultimatums, but through honest conversation and open doors. It is full of respect and of compliments. It's a relationship that is an open book, with no hidden pages. There are no lies, no secrecy. Neither person hides their true feelings from the other person, even when those feelings may hurt the other person. But then there is forgiveness and understanding and a willingness to strive for a better tomorrow (cliche, I know, but get over it. This is my definition). And I don't mean all of this in just the context of a marriage, but also in friendships that have a desire to last a lifetime. Both halves have to have a desire to try to maintain the relationship despite long distances, differences of opinion, and frustration.
Ultimately, "I love you," to me is the same as saying, "I am willing to maintain and develop this relationship despite any and all hardships. I am willing to try with you."