The vertical lines show the six strings on the guitar. The line to the left in the diagram is the darkest/lowest string.
The horizontal lines show the frets on the guitar. Here the first five frets are shown, but if it is a chord higher up on the fretboard it will be shown on the left side of the diagram which fret to start from. The number will also have a yellow background.
The black dots show where to put the fingers on the fretboard, and the numbers inside them show which fingers to use.
The letters D A D F# below the diagram are the notes on the strings that are played.
Above the diagram you can see the symbols 'O' and 'X'.