A fretboard diagram is a picture showing you where to put your fingers on a guitar to play chords.
The black bar at the top represents the nut. The vertical lines represent the strings and the horizontal lines represent the frets. But most diagrams leave off the labels for frets and strings. The circle represents your finger and the number inside the circle tells you which finger to use. An “X” over the string tells you “Don’t strum this open string.” and an “O” tells you “Strum this open string!”
Its not very often you get to use your thumb to play chords but there are some out there. By holding the guitar like a blues player, you can curl your thumb around the side of the neck with the bass strings and hold down a fret or two. Like this one used by Jimi Hendrix:
A barre (bar) chord flattens the first finger on one fret across several strings. They often will include a connecting curve or line between the frets you play with the same finger. To play this, try thinking of your first finger as a capo.