Chord diagrams are a notation indicating the placement of the fretting hand fingers at particular strings and frets. There are two common ways to write a chord diagram. The most common way is to arrange the diagram so that the strings are represented by vertical lines. However, I use chord diagrams with a horizontal orientation to provide consistency with the tablature notation.

Dots on the grid indicate the placement of the fretting hand fingers. For example, a dot on the second line from the top in the second column represents string 2, fret 2. A 0 to the left of a horizontal line indicates an open (unfretted) string. An X to the left of a horizontal line indicates a string that is not played.
Chord diagrams indicate groups of notes that are played simultaneously.
The chord diagrams above are the same three chords shown above in stacked tablature.