Sorry, don't mean to try and tell people what they already know, but just realized that the link I gave (or didn't give &#128512 doesn't explain what I, V, vi etc mean. Basically it's the degrees of the scale. There are 8 notes in Western music-A B C D E F G. Forgetting the intervals between notes (A- A sharp/B flat -B -C- C sharp/D flat -D -Dsharp/E flat-E-F-Fsharp/Gflat-G-Gsharp/Aflat) the degrees of the Key of A for example would then be:

1-A
2-B
3-C
4-D
5-E
6-F
7-G

For the key of D it would be:
1-D
2-E
3-F
4-G
5-A
6-B
7-C

The degrees are written not as 1 2 3 4 5 6 7 but as their Roman numeral equivalents I II III IV V VI VII.

The most commonly used degrees are I (the tonic or home), IV (sub dominant) and the V (dominant)

In a major key the second, third, sixth and seventh are minor chords and are written in lower case to designate this (e.g. ii rather than II)

Chords themselves are - in their basic form- built from 3 notes (triads) in their scale. For Major chords these are the I III and V notes (For C this is C E G).
For Minor chords these are I IIIflat and V (so C Eb and G) for example

Only just really getting to grips with basic music theory myself after years of playing so hope I've got this right!! Certainly still far from comfortable with understanding modes etc but hope this helps a bit