Just another file name to add to archive, I think. Dash as an argument to -f means "direct the output to stdout" (or take input from stdin)
Ofcourse, CRC's man page said it all already.
$ tar cf - -
tar: -: Cannot stat: No such file or directory
$ tar cf - test.c
$ tar cf - test.c
test.c
0100644 0027153 0024461 00000000063 10012711566 012563 0
ustar belousa mkgroup-l-d
main(){
puts(strcat("This is a test", "crap"));
}