Reading Roman numerals
Each chord is labelled by the scale degree it is built on: I for the first degree, V for the fifth… Uppercase means a major chord, lowercase a minor chord, and the small circle marks a diminished chord.
These numerals don’t depend on the key: the same progression, such as I–V–vi–IV, can be played in any key. That is what lets you transpose a song or improvise over a familiar progression.
Finding the key of a song
Most of the time, the first and last chords of a song give away its key. If every chord of a song appears in the list for one key, you have found it.