。
You're supposed to keep your promises.
Literal
Promise [topic-は] keep-thing [copula-だ]-[assertion-よ].
The pattern [dictionary form verb]+ものだ expresses a general truth, expected behavior, or cultural norm — 'that's just how it is,' 'that's what one does.' The final よ adds tone of 'you should know this.' 守るものだよ fixes 'keeping promises' as a default expected norm.