The number "I" means the chord from the first note of the scale (e.g. C E G in C major, or F A C in F major), and uppercase means major and lowercase means minor. Other numbers will then be e.g. "V" will be G B D in C major. You may then add digits as well in which case they indicate intervals above the bass, e.g. "V6" is a first inversion chord (e.g. B D G in C major) and "V7" adds the seventh (e.g. G B D F in C major).