A nice introduction to borrowed chords even though it’s on a piano. 🙂
Borrowed chords often refer to using chords from the relative minor key but it’s often used to indicate chords that aren’t in the current key. The main idea is to approach the G chord with it’s dominant chord (up a fifth interval) which is D7. This is known as a secondary dominant. Note the progression Am D7 G C follows the cycle of fifths!