cheer up!; lighten up!
Casual, friendly encouragement to someone who seems down or discouraged. Often used among friends or peers.
Cheer up, tomorrow will surely be a better day.
Don't make that face, lighten up.
Means 'do your best' or 'hang in there'; 元気出せよ focuses on emotional state rather than effort.
A phrase composed of 元気 (genki, 'energy, spirit') + the imperative form of 出す (dasu, 'to put out'), literally 'put out your spirit'. The sentence-ending particle よ adds emphasis.