It means that you can play chords like you would on the top 4 strings of your guitar, since the harmonic relationship between a D-shape, G-shape and an A-shape will be the same. That was enough to get me started! Just imagine you're playing the top 4 strings of your guitar, and ignore the fact that the 4th string is higher (If you wanna play melodies, just stay on the top 3-strings)