When a yukkuri rub their cheeks with eachother or with a human (usually a human's finger). It's the main way a yukkuri displays and receives affection and is most often done with their parents or human owners in the case of pet yukkuris. It's also used to keep eachother warm during cold weather.
Like human babies, koyukkuris are desperate for affection and is often seen asking their parents for rub-rub. They also tend to attempt to rub-rub anything they come across to be it objects of interest or even random human strangers.
In older comics it's shown that 2 adults rubbing with eachother can cause pregnancy though it rarely happens nowdays. If the action produces a pregnancy, please also use the sex tag.