。
Breaking a promise isn't good.
Literal
Promise [obj-を] break-[nominalizer-のは] not-good.
Verb + のは nominalizes an action into a topic: 約束を破るのは = 'the breaking of a promise, as a topic.' A productive pattern for forming judgments about actions — ~のは大事だ, ~のは楽しい, etc. 良くない is the negative of the irregular adjective いい ('good'), which uses the stem よ- for conjugation.