。
All you have to do is keep quiet.
Literal
Be-silent-[even-さえ] is-staying-[if-ば] is-good [explanatory-のですよ].
~さえ…ば ('if only X / as long as X') is one of the cleanest 'all you need to do is X' constructions in Japanese. Here it wraps around 黙っていれば — 'if only you are staying silent.' The conclusion いい ('it is good / fine') gives 'it would be enough.' Final のですよ adds an explanatory, soft-instructional tone — the speaker is laying out a simple solution.