A progression is simply a group of chords played in a certain key. Names like 1-4-5 come from the bass notes of the chords, which are scale degress of the major scale of the key you are in. Simple example:
C major scale: C D E F G A B
C scale degrees: C=1 D=2 E=3 F=4 G=5 A=6 B=7
C scale degree chords
1 chord = C-E-G
2 chord = D-F-A
3 chord = E-G-B
4 chord = F-A-C
5 chord = G-B-D
6 chord = A-C-E
7 chord = B-D-F
1-4-5 progression in C
C / C-E-G
F / F-A-C
G / G-B-D
That's basically what it is. It gets more in-depth as you experiment with different chords and bass notes, but the principle is very simple and easy to pick up.
