There is a non-binding convention that member should spend 2% of their GDP on *their* armed forces. That way when other nations call for help they can respond. It's a guideline figure though and some countries do it and some don't.
I didn't feel that strongly about this a few years ago but with the Ukraine war demonstrating how unprepared certain NATO members were (ex: Germany), despite their pledges at the 2014 Whales Summit[1], which was essentially preparing for this possibility. I can't help but view Trump's anger about this as fortelling, even if obnoxious. Still, I'm confused why this is getting re-interpreted as an attempt at disbanding NATO, from my perspective the allies who refuse to follow through with their promises are fundamentally what challenges the alliance's integrity, not the allies who call them out on it...
1. https://en.m.wikipedia.org/wiki/2014_Wales_summit#Wales_Pled...