。
She is a very cheerful person.
Literal
She [topic-は] very bright person is.
明るい literally means 'bright' as in lighting, but applied to a person it shifts to 'cheerful, sunny, upbeat.' The same metaphor works in English ('a bright personality'). The opposite pole, 暗い ('dark, gloomy'), follows the same parallel — a person who's 暗い is moody or down. This brightness/darkness mapping is one of Japanese's most productive metaphors for emotional tone.