A lot of musician refer to notes and chords by their ‘number’ refering to their position in the scale.
R, 2, 3, 4, 5, 6, 7
In C that would be
C = 1
D = 2
E = 3
F = 4
G = 5
A = 6
B = 7
So when someone refers to the “6th” that’s the A when in C Major.
Other scales work the same way, just start on a different note, ie. the scale G
G = 1
A = 2
B = 3
C = 4
D = 5
E = 6
F# = 7
The Nashville Numbering System uses numbers as above to denote chords – 1, 4, 5 in the key of G would have chords G Major, C Major and D Major
Some people use Roman Numerals for chords:
ie. In the scale of C Major
I = C Major
ii = D Minor
iii = E Minor
IV = F Major
V = G Major
vi = A Minor
vii° = B diminished
Note the use of capitals to denote major chords and lowercase to denote minor chords.
A note about the blues (and some rock and some country) – the Major chords (I, IV and V) are played as Dominant 7ths (flat 7ths) rather than Major 7ths – which gives that bluesy feel to the progression. According to theory that is playing the ‘wrong’ chords – but theory is just a starting point, and if a chord progression sounds good; play it.
So when someone calls out “I, IV, V” (actually they say “one, four, five in the key of X” – X being whatever key they want to play in. They may also say “Blues in the key of X” which means the same thing.)
The chord structure will be 12 bars - one chord per bar: I, I, I, I VI, VI, I, I V, IV, I, V
In the key of G the chords would be - one chord per par: G7, G7, G7, G7 C7, C7, G7, G7 D7, C7, G7, D7
In some songs, the last chord is the I instead of the V. Also, the second bar may go to the IV chord.
And of course there are other variations – but that structure will cover you for a lot of blues, rock and country.
You may see a chart where the “I” may be an “i” – which means you’re in a minor scale and the ‘root’ is a minor chord.