Meaning
To be worth it / To be worthwhile
Example
A viagem foi cansativa, mas valeu muito a pena quando vi aquela praia.
The trip was exhausting, but it was so worth it when I saw that beach.
Origin
The key to valer a pena is the word pena, which here does not mean "feather" but "suffering," "trouble," or "hardship" — from the Latin poena (penalty, pain). So the expression literally says something is "worth the trouble": the reward justifies the effort and suffering it cost.
Source: Ciberdúvidas da Língua Portuguesa.
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More idioms
Acertar na moscaTo guess correctly / To hit the nail on the headAmigo da onçaA fake/false friendAndar na linhaTo toe the line / To behave properlyArmar um barracoTo cause a public sceneRodar a baianaTo cause a public scene / To go ballisticArrumar sarna pra se coçarTo ask for troubleArrastar as asasTo hit on someone / To flirtAbrir o jogoTo come clean / To open upArregaçar as mangasTo roll up one's sleeves / To get to workAgarrar com unhas e dentesTo hold on for dear life / To seize an opportunity