。
Keeping your body temperature at the right level is an important thing.
Literal
Body-temperature [object-を] appropriate-temperature [at-に] keep [nominalizing-ということ][topic-は] important thing is.
A textbook-style statement of a general truth. 体温を適温に保つ illustrates the ~を~に保つ pattern ('keep X at Y / maintain X in state Y'), where the object is marked with を and the target state with に. The whole verb phrase is then nominalized with ~ということ, a slightly wordier alternative to plain こと that signals the nominalization with a reflective distance — 'the idea/fact of keeping...'. The pattern ~ということは大切なことです is extremely common in didactic or instructional prose, and its repetitiveness (ということ ... ことです) is characteristic of that register rather than a stylistic flaw.