A chord diagram displays the strings, frets, and finger numbers required to play a given chord. Vertical lines represent strings and horizontal lines represent frets. The diagram shows you where to place your fingers and often indicates which fingers play which notes with numbers 1-4 (with 1 being your index finger and 4 being your pinky.)
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If the chord is higher than the first five frets, a roman numeral is used to indicate which fret the chord begins on. An "O" indicates a string is to be played open, and an "X" above a string tells you to skip or leave out that particular string. A chord diagram displays the 6th (thickest) string on the left and 1st (thinnest) string on the right, as if you were sitting across the dinner table from your guitar neck.