。
A contented heart is a perpetual feast.
Literal
Satisfied heart [topic-は] eternal's feast [copula-である].
An aphorism on contentment. 満ち足りた ('fully satisfied') is the past/resultative form of 満ち足りる, acting as an attributive — 'the heart that has become fulfilled.' 祝宴 ('celebratory banquet,' 'feast') is a formal/literary word, raising the tone. Closing with the formal copula である maintains the aphoristic register typical of proverbs.