17 Grk “a son of peace,” a Hebrew idiom for a person of a certain class or kind, as specified by the following genitive construction (in this case, “of peace”). Such constructions are discussed further in L&N 9.4. Here the expression refers to someone who responds positively to the disciples’ message, like “wisdom’s child” in Luke 7:30.
18 The response to these messengers determines how God’s blessing is bestowed—if they are not welcomed with peace, their blessing will return to them. Jesus shows just how important their mission is by this remark.

In other versions

And if a son of peace is there, your peace will rest upon him. But if not, it will return to you.

If someone who promotes peace is there, your peace will rest on them; if not, it will return to you.

If a person of peace is there, your peace will rest on him; but if not, it will return to you.

If those who live there are peaceful, the blessing will stand; if they are not, the blessing will return to you.

And if a son of peace is there, your peace will rest on him. But if not, it will return to you.

And if the son of peace be there, your peace shall rest upon it: if not, it shall turn to you again.

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