If you are not farmiliar with the term “tell” its a term used in cards and poker. It is a mannerism or a phrase that gives away or “tells” what the person is trying to hide. You can see the usefullness of being able to read the “tell” of the person you are playing against in polker. If he is trying to hide the fact that he is holding a winning hand or has nothing and is bluffing, and you can read his “tell” whether it be a mannerism, a phrase, an eyeblink, sweating, etc you can use that to your advantage.
If you can read Trump’s tell it can save you a ton of time listening to useless and misleading information.
In Trump’s case if you hear him say “That’s what I hear” of “People tell me” you know everything that follows that will be either a lie or misleading and you can just ignore what follows.