By convention in music, we use Roman numerals to signify what chord we should play relative to the root (key). "I" refers to the root/tonic/key and we count up from there. [1]

So, for example, a common three chord progression in a major scale would be I – IV – V. If we take the key of C, those would be C, F, G, as F and G are the fourth and fifth chords respectively.

In the key of G, it'd be G, C and D. In that key, a good example song is "Sweet Home Alabama", where almost the entire song is just V - IV - I over and over again.

One of the most popular chord progressions, used in an astounding number of pop songs is known as the "Four Chord Trick", I – V – VI – IV, famously demonstrated by the Aussie comedy band Axis of Awesome[2]

I think I'd agree with the person you're replying to, both in that the original source is flawed due to not including the "dupes", despite them being important, and also because key is largely irrelevant, chord progression is much more important.

[1]https://en.wikipedia.org/wiki/Roman_numeral_analysis [2]https://www.youtube.com/watch?v=oOlDewpCfZQ.

Minor chords are written in lowercase so the Axis of Awesome progression should be I-V-vi-IV

This is about the simplest description of chord progressions you're going to find.

There is something peculiar that people who understand music theory tend to have a difficult time explaining it without stacking concepts and new terms.

While I'm sure those concepts are necessary for completeness, to a beginner in becomes a brick wall, and this is blessedly direct compared to, to e.g., the linked wikipedia entry.