The chord diagrams on this website contain a lot of information that can easily be confusing. Here's how to read them.
Table of contents
X means that the corresponding string should not be played. O means that the corresponding string should be played without fretting the string (playing the open string).
The low E string is on the left and the high E string is on the right.
The number in the circle indicates which finger you should use to fret a string. The following picture shows which finger corresponds to which number.
The letter below a string indicates which note is currently fretted.
The number below a letter indicates the interval which is currently fretted. The interval is the distance between the root note of a chord and the current note, but more on this in the introduction to intervals.
In contrast to the diagram on the left, the diagram on the right does not start at the first fret, but at the third fret. This is indicated by the number (3.) on the left side of the diagram.