。
Get on with your work in silence.
Literal
Be-silent-[and-て] work [object-を] do-[command-なさい].
黙って is the te-form of 黙る functioning adverbially — 'silently / in silence.' This is the standard way Japanese turns a verb into a manner modifier: take the te-form and stack the main verb after it. Paired with the ~なさい command, it's a brisk instruction to stop talking and work.