[FDOM2008.9] what's this black band on Home bottom?

[herve5.freerunner at ouvaton.org]

Of course it's only another awful newcomer question...

I just bought a FR and installed FDOM on it, I'm happily testing TangoGPS
among others, but I can't understand what is this black overlay at the
bottom of the Home screen (semitransparent, with a star and two "+"
characters on it...)

How do I use it?

TIA!
Hervé