Does it mean that if you think of someone you have met, and can't stop thinking of them, you have fallen for them?

This depends on many things!

First on you age: If you're a teenager of around 14 or something,  then that's what will happen with you, because even I've experienced the same thing,  but that doesn't mean that you have fallen in love with them!

Second, if you always and everywhere think of that person then yes you like that person or are in love with them!