To pep someone up; to cheer someone up or encourage them.
彼女は試験に落ちた友人を元気づけるために手紙を書いた。
She wrote a letter to cheer up her friend who failed the exam.
先生は生徒たちを元気づけようと、クラスで褒め言葉をかけた。
The teacher tried to encourage the students by giving compliments in class.
小さな成功体験が人を元気づけることもある。
Small successes can sometimes cheer people up.