。
If you make a promise, you should keep it.
Literal
Promise [obj-を] make-[cond-たら] keep-[should-べきだ].
Conditional ~たら setting up a consequence, with obligation ~べきだ. A classic aphorism with the implicit object of 守る (the promise) carried over from the first clause — Japanese routinely omits re-mentioning referents that were just introduced.