When someone says "I'm falling for you," they're usually expressing that their feelings for you are deepening. They're moving beyond simple attraction or friendship, and beginning to feel more intense emotions that are often associated with romantic love.
It's a way of saying that they're developing strong affection and care for you, and they're starting to imagine a potential future with you. This phrase often indicates that the person is moving from liking you to loving you. They're in the transitional phase, hence the term "falling."
However, the exact meaning can vary somewhat depending on the context and the individuals involved. It's always best to have open and clear communication with the other person to fully understand their feelings and intentions.