Describes something located or moving at a great height in the sky. Often used with verbs like 飛ぶ (fly) or 昇る (rise).
Birds are flying high in the sky.
Fireworks were launched high into the sky.
Compound of 空 (そら, 'sky') and the adverbial form 高く (たかく) of 高い (たかい, 'high').