Based on the 'floating' installation method it depends on the type of underfloor you have what kind of underlayment you need for your wooden floor. (Floating means: wooden floorboards not fixed to the underfloor with adhesive or nailed onto joists/on existing floorboards or sheetmaterial, and where the T&G is glued with PVAC woodglue.)
Concrete floors = DPM plus sound-insulation (DPM stands for Damp Proof Membrane). More and more sold as a 1 in 2 product with handy self-adhesive overlapping strip of DPM. (Blue roll in picture)
The DPM layer prevents any residue moist from the concrete getting into the wood. Not really needed on concrete underfloors on second floors, but it won't hurt.
The attached foam layer acts as an sound-insulation preventing your real wooden floor 'sounding' like a Melamine Laminate floor and has the ability to 'fill' some minor unevenness in the underfloor.
In existing floorboards, plywood or chipboard underfloors DON'T us DPM. This can cause condensation between the DPM and underfloor/wood. When your underfloor is level a 2-3mm foam is sufficient (white roll). If you need more sound-insulation it's best to use a thicker underlay (brown roll in picture, also available with DPM attached to it for concrete floors).
Whatever kind of underlayment you need, don't cheapskate with the materials you use: it'll cost you in the end (inferior materials = more time corecting plus the end result of your natural real wooden floor could sound like a 'plastic' Melamine Laminated floor).
If you have two or more different types of underlayment, read here