The comedy group Axis of Awesome popularised the "Four Chord Song": a sketch stringing dozens of pop hits over the same four chords. Behind the joke is a real fact — the I-V-vi-IV progression (C-G-Am-F in the key of C) underpins a huge share of pop music from the last sixty years. On guitar, it is usually adapted to the key of G to avoid the barre chord.
The 5 chords that cover pop
Starting in the key of G major (the most beginner-friendly), the progression becomes G-D-Em-C — four open chords with no barre. Add Am for variants in the key of C.
The I-V-vi-IV progression in practice
In the key of C major: C-G-Am-F. The F barre chord is the classic beginner roadblock. Tip: in the key of G major, the same progression becomes G-D-Em-C — no barre — the backbone of an overwhelming majority of folk hits.
A few songs that use this progression
Some famous examples: Let It Be by The Beatles (C-G-Am-F), Someone Like You by Adele (A-E-F♯m-D), No Woman No Cry by Bob Marley (C-G-Am-F). Once you know the pattern, you start hearing it everywhere.
How to practise
Work on transitions on a loop: G→D, D→Em, Em→C, C→G. Once they are fluid, play them in a shuffle — the order changes, but the chords stay the same.