An Added 6th chord or simply a 6th chord has a major triad plus a major sixth interval.

 

For instance, a C-Add6 or C6 chord has the notes C, E, G, A, a G6 chord has the notes G, B, D, E, and so on.  

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The Swars are Sa, Ga, Pa and Dha.

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A 6th chord gives you the feel of a major chord combined with its relative minor chord. For instance, a G chord has the notes G, B, D. Its relative minor chord is Em which has the notes E, G, B, and a G6 chord contains the notes G, B, D, E, so think of it as a combination of G and Em chords.

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Another interesting thing about a 6th chord is that if you invert the chord and start from the last note in the chord, it becomes a minor 7th chord based on that last note. So, in the above example a G6 chord and an Em7 chord have the same notes but a different root note or starting point. So, they can be substituted for each other but the feel will be different.