You can fall in love more than once. I think the whole concept of "love" when we are talking about significant others is overplayed in today's society. There is no "the one".
You can also fall in love and then fall out of love with the same person as well. Happens all the time.
But overall it is having someone who you are attracted to but at the same connect to on a deeper psychological level as well, but there must be some sort of elapsed time in the relationship. And I don't mean you necessarily have the same interests as him/her, but you are both calibrated mentally without any effort.
And I don't want to use that same cliched advice, but the only way to know you connect with someone like that is to "be yourself". What that means is you're not trying to get their approval or supplicate to them, you're not trying, you just are.
Even after you connect with someone like that, it doesn't necessarily mean you are in love...
Like someone said above, love at first sight is **. It takes time to create any kind of bond. You being attracted to someone for looks =/= being in love. It's obvious but a lot of people fall into this trap.
Looks are ofc the first qualifier, whether we deny it or accept it. It's just basic biology. All the other qualities you look for come after.