This is the 348th most frequent French word.
pleine
"Full" or "filled."
Here, 'pleine' is used as an adjective agreeing in gender and number with 'salle' (feminine singular), meaning 'full'.
La salle est pleine de monde.
The room is full of people.
In this sentence, 'pleine' acts as an adjective describing 'lune' (feminine singular), denoting its state as 'full' or 'entire'.
Elle a marché sous une lune pleine.
She walked under a full moon.
'Pleine' here is used metaphorically in a phrase to describe an abundance of energy; it does not describe the subject directly but an aspect of their state.