What exactly is chord progression?

It's the same whether it's for guitars, pianos, or any instrument that can play chords. If you are playing something in the key of C, then C is the I chord; F is the IV chord; G is the V chord. There could also be II chords, III chord, vi chords, etc. Capital Roman Numerals are used for major chords; lower case is used for minor chords. Chords "progress" during a piece from the I chord to other chords, or actually a piece could start with another chord but would end on the I chord. The tonic chord is the I chord. In the key of G, the tonic chord is G, the IV chord would be C, and the V chord would be D. The subtonic is a whole step below the tonic; in C, it would be Bb, and in G, it is F. All songs that I ever heard have chord progressions.