Lots of different things.
There's family love which is basically shouting and complaining all the time except for those magical moments where you all get along and you always know that no matter how much you dislike each other at times you will always love each other deep down inside and you'd do anything for them. Family love is knowing that if someone upset you then they'd stand up for you and punch that sucker in the face for so much as trying to make you sad.
There's friend love which is where you can joke around and muck about till the cows come home but you can always talk to each other about anything and everything. Friendly love is missing that one person who can make you laugh when you don't even want to smile, that person who will except who you are, everything you've done and will greet every insult you throw their way with a better insult.
Then there's love love. That sort of love you don't glide gracefully into, you fall face first into it. You can dislike that person with every ounce of your heart sometimes because they get on your nerves but in the end, when push comes to shove you will always love them. If they ever do something to upset you it won't just hurt you mentally it will hurt physically. But it is worth falling in love to have that one person who completes your soul, to wake up to the face of the only other human being in the universe who loves you good, bad and ugly. They don't care about your imperfections because to them all of your good things and bad things make you who you are and without every last one of those things you aren't you.