It is a ground connection (usually) and a heat sink. The Sparkfun page covers these in one of the tutorials. Be sure to read through the comments at the end.
73
-Jim
NU0C
On Sun, 27 Dec 2020 16:18:28 -0500
"stefan baartman" <seb21051@...> wrote:
Is the slug just a ground connection?
Jim Shorney