[aussiblue]

Bumper apron or valance. As they say at: https://www.carid.com/articles/what-...r-valance.html

In some cases, valance panels don't enhance aerodynamics at all. They might be simple cover pieces that conceal areas below the level of the bumper (or bumper cover) that would otherwise be exposed. In the front, this would effectively cover and protect the lower part of the radiator. more details on - https://www.carid.com/articles/what-...r-valance.html

I think the bolded bit is mostly what it does although the air flow might be untidier without it.