When someone says that a human is a homo sapien what are they referring to?

Homo sapiens means wise man, thinking man, rational man, knowing man.

It is the species name for us, humans, sometimes referred to as anatomically modern humans or AMH. Our trinomial name is homo sapiens sapiens . The skeleton taxonomy is :-

• Domain Eukaryota
• Kingdom Animalia
• Phylum Chordata / ( sub P. Vertebrata )
• Class Mammalia / ( sub C. Theria )
• Order Primates
• Family Hominidae
• Genus Homo
• Species H. sapiens / ( sub S. H.s.sapiens )

You can find the full taxonomy on Wikipedia under "homo sapiens"
It should be "homo sapiens". It means "wise man" and it's the Latin name of our species - i.e. humans.
It combines the Latin Word homo (mankind) with the adjective sapiens (wise, sensible, understanding) to describe the modern human race. The implication is that all previous forms of humankind were stupid, which is a completely false idea.
It means 'wise man', or 'learned man'.