The one I IV V in music refers to chord changes. Know that everything in music is relative.

If you are playing a I IV V in the key of A the chords would be A, D, and E.

A is 1. B is 2. C is 3. D is 4. E is 5. It’s fairly simple. You just have to count from the established “one” chord.

Sometimes though, you have to know when to apply a sharp or flat.

A I IV V in the key of F, for example, is not F B and C but F B flat and C.

Your average blues tune is a I IV V pattern. Because the dominant 7th chord sound is the essence of blues, the chords would be I7 – IV7 – V7. So, for example, in the key of A the chords used are A7, D7, and E7.

Here is a chart of each key’s I IV V pattern starting in the key of C and moving up in fifths.
This is good to know for any musician and a must know for blues players.

I IV V
C F G
G C D
D G A
A D E
E A B
B E F#
F# B C#
Db Gb Ab
Ab Db Eb
Eb Ab Bb
Bb Eb F
F Bb C