This is the 4352nd most frequent Russian word.
больно
Painfully; it hurts.
In this sentence, 'больно' modifies 'падать' (to fall) and describes a physical sensation of pain associated with the action.
Мне было больно падать.
It was painful for me to fall.
Here, 'больно' is used metaphorically to express an emotional discomfort rather than physical pain.
Больно смотреть на происходящее.
It's painful to watch what is happening.
In this case, 'больно' is used to indicate strong emotional pain caused by something said.