6 awg is the wire guage. thhn is the insulation type as is thwn-2. This wire is copper and needs to be placed in a raceway/conduit between termination points.. the insulation rating thwn-2 allows the wire to be in a wet location inside conduit. One word of caution do not use the 90C ampacity rating only 75C or 60C.
http://www.cerrowire.com/files/file/SLiPWire THHN(10).pdf
http://www.cerrowire.com/files/file/SLiPWire THHN(10).pdf