complete change of mood; refreshing one's spirits
Used for a thorough mental or emotional reset, often after a break, change of scenery, or new activity.
I felt completely refreshed after taking a walk.
I'm aiming for a complete change of mood by rearranging the room.
リフレッシュ is a more casual, everyday term for refreshing oneself, while 気分一新 implies a more thorough or dramatic change in one's mental state.
Compound of 気分 (mood, feeling) and 一新 (complete renewal). The exact historical derivation is uncertain, but the meaning is transparent from its components.