@Sac Here’s another nice video, covering how to inverse simple triads, and add some bonus spice.
When he talks about “chord functions”, he means how the chords would function in a progression along with other chords within the key. This is where chord symbols and Roman numerals come in. Every scale/key (the terms are basically interchangeable) has a number of chords associated with it, labeled 1 through 7; these are the scale degrees. Each of these has a specific function within the scale, and are called a bunch of different names. Here’s a chart for the C major scale.
Here, you can also see the Roman numerals (at the bottom) associated with the different degrees of the scale. It might seem confusing, but it’s actually pretty logical. The capitalised letters mean that the triad is a major one, and the lowercase ones indicate that the triad is a minor one. The last one with the small circle, vii°, indicates a minor triad, that is also diminished. Diminished chords sound “unresolved” and can be used to create tension in a progression. The reason it’s called the “leading tone” is because it resolves nicely back to the tonic, or 1 chord. Every major key will have the same Roman numerals, regardless of which key you’re in, and the functions within a chord progression remains the same. This is a bit more advanced stuff, since you of course have to know how to create the chords, but it’s incredibly useful for musicians who know this, because then, you can write down a whole chord progression for a song, without specifying the key. You’ve probably heard a “2-5-1” progression being mentioned, a common thing in jazz music; this is what they’re refering to.
There are a bunch of other symbols you can add too, to specify which type of chord you mean, if it’s not a regular major or minor triad.
Here’s a list of a few common chord progressions, with their associated Roman numerals.
And here’s a list of songs that use the I–V–vi–IV progression, one of the most common ones in pop music. Some of the songs modify this progression slightly (mainly by starting at a different position in it), as indicated in the chart, but they still mostly sound the same. So, people saying that “all pop music sounds the same” are not very far off from the truth.