What does an asterisk "*" mean in hexdump output?

A line in the hexdump output consisting just a * means same as the line above. This is mentioned in the hexdump's manpage at the -v option (easy to be overlooked).

As mentioned in the comments, without to verbose option to hexdump, -v, the asterisk indicates "same as above". Most likely seen on empty files or parts thereof:

> hexdump -C -s 12 -n 32 test-data/blk00000.dat
0000000c  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
0000002c

Adding the option -v to hexdump gives the full result, which is especially useful, if the output is used as input to some other process:

> hexdump -v -C -s 12 -n 32 test-data/blk00000.dat
0000000c  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
0000001c  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
0000002c

(Answer added for completeness)