Here’s what Professor Hallux has to say:
The first part of this question is easy – it’s called your philtrum.
Humans aren’t the only ones to have some sort of crease in this place. Many mammals have them although they come in all sorts of shapes and sizes.
For some animals, it’s a way to help moisture travel between the mouth and the nose. Moisture in the nose is particularly important to animals who rely on a good sense of smell – like dogs!
However, our philtrums don’t do a job at all – they’re just left over from our evolution at a time when we perhaps relied more on a sense of smell to survive.