Basically having a wank, jerk, or male masturbation.
Your erect penis represents the banjo "shaft" and the hairs represent the strings. Your ball-bag/nuts/scrotum representing the resonant body of the banjo.
Play away and try not to get too carried away or else you might.... Ooops.... Sorry... I've just cum.
Based on the unorthodox sexual practices enjoyed by Bill Pullman's character Detective Harry Ambrose on the show The Sinner, a Harry Ambrose is when you ejaculate on your partner's stomach, pull out a chunk of their pubic hair and in an Emeril Lagase style manner, throw the pubes onto said ejaculate. The act is only complete if after the pubes are placed that you step on your partner's fingernails until they turn black and/or purple.