Think of the algebraic formula:

Y=5X+19

Y and X are variables, but 5 and 19 are numbers. The numbers don’t change.

In Algebra, we use letters as variables, and think of digit based numbers as the actual numbers.
Here are the first four chords of a standard jazz tune

Fm7 | Bb-7 | Eb7 | Abmaj7 |

Suppose your singer can’t reach the low notes, and wants to transpose it up a full step. You end up with:

Gm7 | C-7 | F7 | Bbmaj7 |

This works because the relationship between the chords is the same in both keys. To represent this relationship, we represent the chord using roman numerals. If we assume the first version is in the key of Ab, and the second in the key of Bb, we say the last chord of the sequence is the root, and refer to it using the roman numeral I, and will call it “the One”. We use lowercase letters to refer to minor chords. So the sequence becomes:

vi-7 | ii -7 | V7 | Imaj7

Thus, the letter version is the “actual” chords, whereas the numeric version is the variable.