Originating as a predominantely British term for masturbation, to wank is to play lead guitar that's basically equivalent to masturbation; characterized by notes in the very high end of the scale being played super-fast and repetitiously, almost always in the metal genre. Generally used in a derogatory way.
Can be used as a verb or a noun, so the lead guitarist can also be called a wank (or wanker if you prefer) and his playing would be wanking.