。
Keeping promises is a very important thing.
Literal
Promise [obj-を] keep-[nominalizer-こと][topic-は] very important thing [literary-である].
Uses the literary copula である instead of the plain だ, giving this a written/formal flavor typical of essays, editorials, and academic prose. である is a contraction of で+ある, making it more analytical in structure than the fused だ. The omission of an explicit subject is also typical of generalizing prose.