Sp. is an abbreviation of species. However, it has a specific meaning when used in a species name. In binomial nomenclature every animal living or dead is supposed to be given a genus and a species name; however, in some cases we aren't sure exactly where a species fits in the system, so we have to use a metasyntactic variable. In this case we will write Agenus sp. In some cases the form Agenus aspecies ssp. is also used for an unknown subspecies.
There is some potential for confusion in sp. and related abbreviations: the plural of sp. is spp.; subspecies is ssp., and the plural of subspecies is sspp. All of these can be used as metasyntactic variables in scientific names.