Matthew 10:27

“What I tell you in the darkness, speak you in the light, and what you hear in the ear, proclaim upon the housetops.”

So what He is telling them ‘in the darkness' they must speak out in places where all can see, and what He as it were whispers in their ear they are to yell out from the housetops. For that indeed is the purpose for which He has called them. It is in order that they might be His witnesses. News was regularly literally shouted from high housetops so that it could reach as many as possible.

The reference to darkness and light looks back to Matthew 4:16. ‘The people who sat in darkness have seen a great light, and for those who sat in the region and shadow of death life has sprung up.' Jesus here confirms that Matthew has taken that idea from His own teaching (as well as from Isaiah 9:2). His disciples had been in darkness, but He has come as a light to speak to them in the darkness (compare John 3:19) so that they might become a light to others (Matthew 5:14; Matthew 5:16). As the light of the world (John 8:12) He has spoken to them in the darkness, so that they might be filled with light (Matthew 6:22).

The ‘hearing ear' is also a favourite idea of Jesus (Matthew 13:16, contrast Matthew 13:14. See also Matthew 13:43; Luke 14:35; and compare Mark 4:18; Mark 4:20; Mark 4:24; Luke 8:18). What you hear in the ear is an indirect way of saying ‘what God has said in your ear' in a similar way to the divine passive.