A 3D printer nozzle or extruder is the tip of a 3D printer that extrudes the heated filament onto the print board. On the ender-3, you can install different size nozzles like 0.2mm. Mine is the default size 0.4mm. Some filaments require bigger or smaller nozzles for printing, but I haven't tried too many other filament types besides PLA filament. But I might use more in the future.
(Above clip taken from matterhacker)