Translation:Do it quietly.
@LewisH65 I am sorry but You are not correct. 조용히 = 조용하게 = quetly (adverb). Those two are absolutely the same.
조용하다 = quiet (adjective)
Isn't "Be quiet" = 조용하십시오 ? (Imperative form as per "Formal Md." lessons).
The "Tips and notes" for this lessons puts 조용히 and 조용하게 as being the same thing, if I understood it correctly.