Short for the Latin confero (the present active form of conferre), meaning 'confer' or 'compare'. It is used in biology and taxonomy to indicate that a new species is very close to, and may in fact be, an already known species.
This may be done because a specimen is incomplete (common in the study of fossils) or because the initial determination is intended to be temporary, pending further study. It would be written as Agenus cf. aspecies. Sometimes you will also a see a cf. in front of a genus name, but higher taxonomic levels are listed as incertae sedis instead.
You should be careful not to confuse cf. with aff., which indicates that the new species is related to a known species, but that the author is fairly certain that they are not the same species.