ELI5: Why can a bridge holding thousands of tons of cars be more at risk from a small group of soldiers marching in sync than from all that weight?

Fell into a 2am rabbit hole about the Tacoma Narrows bridge collapse and learned that armies are actually trained to break their marching rhythm when crossing bridges. That completely broke my brain. Like engineers will spend years and lots of government money building something that can support insane amounts of weight but a couple hundred guys walking in a coordinated rhythm could potentially mess with it more than all that load combined? What is actually happening physically at that point, is the bridge literally vibrating itself apart? How does frequency do something that raw weight cant? I was literally playing on my phone and started scrolling afterwards and fell on this rabbit hole that straight up amazed me.