This phrase expresses regret or a wish that one had done something differently in the past. It means 'I should have done...' or 'I wish I had done...'. Connect it to the conditional ば-form of verbs, i-adjectives, or na-adjectives/nouns.

過去の行為に対して後悔や反省を表します。「〜すればよかった」のように使います。