。
Be ready for anything.
Literal
All-things preparation do-in-advance [imperative-ておけ].
A gruff, commanding sentence. 万事 ('ten-thousand things,' 'all matters') is a formal idiomatic 'everything.' 用意しておく combines 用意する ('prepare') with the ~ておく auxiliary ('do in advance, for future use'). The plain imperative ~おけ is the short, direct command form of おく — masculine, blunt, often military or authoritative.