Understanding Eyepieces and the Focuser

[Gflight]

The scope I am looking at (12" Skyline Dob) has a 2" dual-speed crayford focuser.

It comes with a 9mm Plossl (1.25") and a Erfle 30mm (2").

It has in the kit a 35mm 2" extension adapter which I am unsure of its use.

Does it have to be in for one of the eyepieces to focus or would this be for a camera or something else?

I am thinking of purchasing a Barlow to expand my lens choices and not knowing how they work together I am unsure the Barlow would work.

[Ricochet]

1 hour ago, Gflight said:

It has in the kit a 35mm 2" extension adapter which I am unsure of its use.

Does it have to be in for one of the eyepieces to focus or would this be for a camera or something else?

You might find some or all eyepieces require the extension. When you come to use the telescope it will become obvious whether it is needed or not. 

You generally wouldn't use a camera with a manual dobsonian. In theory you can use such a scope for planetary photography but if you were to get into planetary photography seriously I think you would soon buy a scope that can track by itself. 

1 hour ago, Gflight said:

I am thinking of purchasing a Barlow to expand my lens choices and not knowing how they work together I am unsure the Barlow would work.

You put the barlow into the focuser, and then put the eyepiece into the barlow. This approximately increases the magnification by the factor given on the barlow. 

The type of barlow that would be most useful with your scope would be a 1.25" 2x barlow. 2" barlow and eye piece combinations can be very heavy and it is better to just buy intermediate magnification 1.25" eyepieces. With the two supplied eyepieces a 1.25" 2x barlow would only be usable with the 9mm. Given the focal length of your telescope, this could easily result in magnifications that are above the limits that the atmosphere allows in your location. An intermediate eyepiece between the 9mm and 35mm eyepieces is probably more useful as a first additional purchase.