As this answer says, you first explain the consequences in ways relevant to the manager -- compliance, sales impact, bad publicity, ethics, or whatever applies in your case. In addition, though, you should be able to explain the technical aspects, at least at a high level, to anybody from your peer to your manager to your Uncle Ted who still uses a VCR to watch TV shows.
For example, here's how Wikipedia's page on MitM attacks begins:
In cryptography and computer security, a man-in-the-middle attack (often abbreviated MitM, MiM attack, MitMA or the same using all capital letters) is an attack where the attacker secretly relays and possibly alters the communication between two parties who believe they are directly communicating with each other.
(Wikipedia is a good place to look for accessible explanations of specialist topics.)
Understanding that doesn't require any technical knowledge; you're explaining that there is, as the name implies, a "man" (agent) in between the user and the server he thinks he's communicating with. You can then go on to use an example to explain what happens and how the attacker gains access (as that Wikipedia page does). Start high-level and let the manager's questions guide what else you say; he probably doesn't care about the details of encryption algorithms or DNSSEC or certificate pinning, so don't lead with the technical details. (If he does care, he'll let you know.)
The reason you are having this conversation at all is probably that you need him to make a decision. Give him the tools to make that decision, be ready to provide additional information if he asks (or clearly needs it, e.g. because he's misunderstood something), but help him focus on his problem, not yours.