chr5 156479568 . C CGTT,*
what does * mean?
As far as I can make out it means that at that position, there is neither a C or a CGTT - so basically there's a deletion of the C allele. It wouldn't surprise me if the vast majority of these kind of compound-null loci are generated in error - and there is no variant locus there at all - or there is but the genotyper can't distinguish the alleles properly.
Have a look here for a nice explanation