What is G92 used for in G-code
The G92 command is used to set the start position (origin) of one of more axes (including the current extruder) to any arbitrary value. The command G92 E0 is often used to perform retraction and nozzle priming. For example, the following commands are often used in start-gcode sequences (prologues) to prime the current extruder by extruding a small amount of filament:

G92 E0     ; Reset the extruder's origin
G1 F200 E3 ; Extrude 3 millimetres of filament
G92 E0     ; Reset the extruder's origin


RepRap Wiki: G92: Set Position