“Ikki-ichiyū一喜一憂” means alternating between joy and sorrow with every small change - being overly emotional about ups and downs.
It describes someone who gets too excited or depressed by minor events.
EX.
「テストの点数(てんすう)で一喜一憂(いっきいちゆう)するのはやめなさい!」
"Stop getting excited about test scores!"