Assimilation tubules are fine tubes that extend from a Borg drone's hand or arm to pierce a victim's skin and inject nanoprobes.
The tubules are the means by which drones begin individual assimilation. With a quick strike they penetrate the skin, usually at the neck, and deliver the nanoprobes that transform the victim into a drone.
The process was shown during the Borg attack on the Enterprise-E in 2373, when crew members captured by the Borg were assimilated in the corridors, and it remains one of the most recognizable images of the Collective.