気(き)が晴(は)れる means to feel better, to feel as if a weight has been lifted from one’s shoulders. Your 気 (mood, feelings) does the action of 晴れる (clearing up).

Examples of 気が晴れる

Further Reading

• Garrison, Jeff et al. Kodansha’s Dictionary of Basic Japanese Idioms, Kodansha, 2002, p. 233