It’s posturing to pressure Canada into becoming part of America, because we have a wealth of natural resources, trade route access (see Greenland, Panama Canal threats), and space they’d rather not spend on. Not much different than the precursor to what Russia has been trying to do with Ukraine (and other regions), and Israel with Palestine. Create an environment economically and through social engineering (botfarms, media shills) that makes the public consider annexation and subsumption as a beneficial outcome.
And it’s not really Trump saying it. He’s just the mouthpiece and signatory.